bug in Gadget frontend
<!--To help us understand and resolve your issue, please fill out the form to
the best of your ability.-->
<!--You can feel free to delete the sections that do not apply.-->

### Bug report

**Bug summary**

<!--A short 1-2 sentences that succinctly describes the bug-->
Error message when using yt.load outputs a different file name then what it is actually called. [Here is my data file](https://upenn.app.box.com/s/0108qrvk66pdb9ckyi41fhqnp7s6bxv6)-- it could be that  testing failed for this type. This code worked for one of the provided datasets on the yt-webpage.
(edit by @neutrinoceros : the sample dataset in question is ` snapshot_010 `)

**Code for reproduction**

<!--A minimum code snippet required to reproduce the bug, also minimizing the
number of dependencies required.-->

<!-- If you need to use a data file to trigger the issue you're having, consider
using one of the datasets from the yt data hub (http://yt-project.org/data). If
your issue cannot be triggered using a public dataset, you can use the yt
curldrop (https://docs.hub.yt/services.html#curldrop) to share data
files. Please include a link to the dataset in the issue if you use the
curldrop.-->

```python
import os.path
from os import path
print ("Is it File? " + str(path.isfile('snap_000.11')))
data_ds = yt.load('snap_000.11')
hc = HaloCatalog(data_ds=data_ds, finder_method='hop')
hc.create()
```

**Actual outcome**

<!--The output produced by the above code, which may be a screenshot, console
output, etc.-->

```
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-89-79dabe25cdb0> in <module>
      4 data_ds = yt.load('snap_000.11')
      5 hc = HaloCatalog(data_ds=data_ds, finder_method='hop')
----> 6 hc.create()

//anaconda3/lib/python3.7/site-packages/yt/analysis_modules/halo_analysis/halo_catalog.py in create(self, save_halos, save_catalog, njobs, dynamic)
    335 
    336         """
--> 337         self._run(save_halos, save_catalog, njobs=njobs, dynamic=dynamic)
    338 
    339     def load(self, save_halos=True, save_catalog=False, njobs=-1, dynamic=False):

//anaconda3/lib/python3.7/site-packages/yt/utilities/parallel_tools/parallel_analysis_interface.py in barrierize(*args, **kwargs)
    299     def barrierize(*args, **kwargs):
    300         if not parallel_capable:
--> 301             return func(*args, **kwargs)
    302         mylog.debug("Entering barrier before %s", func.__name__)
    303         comm = _get_comm(args)

//anaconda3/lib/python3.7/site-packages/yt/analysis_modules/halo_analysis/halo_catalog.py in _run(self, save_halos, save_catalog, njobs, dynamic)
    404         if self.halos_ds is None:
    405             # Find the halos and make a dataset of them
--> 406             self.halos_ds = self.finder_method(self.data_ds)
    407             if self.halos_ds is None:
    408                 mylog.warning('No halos were found for {0}'.format(\

//anaconda3/lib/python3.7/site-packages/yt/analysis_modules/halo_analysis/halo_finding_methods.py in __call__(self, ds)
     42 
     43     def __call__(self, ds):
---> 44         return self.function(ds, *self.args, **self.kwargs)
     45 
     46 def _hop_method(ds, **finder_kwargs):

//anaconda3/lib/python3.7/site-packages/yt/analysis_modules/halo_analysis/halo_finding_methods.py in _hop_method(ds, **finder_kwargs)
     49     """
     50 
---> 51     halo_list = HOPHaloFinder(ds, **finder_kwargs)
     52     halos_ds = _parse_old_halo_list(ds, halo_list)
     53     return halos_ds

//anaconda3/lib/python3.7/site-packages/yt/analysis_modules/halo_finding/halo_objects.py in __init__(self, ds, subvolume, threshold, dm_only, ptype, padding, total_mass)
   1471         self._data_source = ds.all_data()
   1472         GenericHaloFinder.__init__(self, ds, self._data_source, padding,
-> 1473                                    ptype=ptype)
   1474         # do it once with no padding so the total_mass is correct
   1475         # (no duplicated particles), and on the entire volume, even if only

//anaconda3/lib/python3.7/site-packages/yt/analysis_modules/halo_finding/halo_objects.py in __init__(self, ds, data_source, padding, ptype)
   1209         ParallelAnalysisInterface.__init__(self)
   1210         self.ds = ds
-> 1211         self.index = ds.index
   1212         self.center = (np.array(data_source.right_edge) +
   1213                        np.array(data_source.left_edge)) / 2.0

//anaconda3/lib/python3.7/site-packages/yt/data_objects/static_output.py in index(self)
    447                 raise RuntimeError("You should not instantiate Dataset.")
    448             self._instantiated_index = self._index_class(
--> 449                 self, dataset_type=self.dataset_type)
    450             # Now we do things that we need an instantiated index for
    451             # ...first off, we create our field_info now.

//anaconda3/lib/python3.7/site-packages/yt/geometry/particle_geometry_handler.py in __init__(self, ds, dataset_type)
     37         self.directory = os.path.dirname(self.index_filename)
     38         self.float_type = np.float64
---> 39         super(ParticleIndex, self).__init__(ds, dataset_type)
     40 
     41     @property

//anaconda3/lib/python3.7/site-packages/yt/geometry/geometry_handler.py in __init__(self, ds, dataset_type)
     48 
     49         mylog.debug("Setting up domain geometry.")
---> 50         self._setup_geometry()
     51 
     52         mylog.debug("Initializing data grid data IO")

//anaconda3/lib/python3.7/site-packages/yt/geometry/particle_geometry_handler.py in _setup_geometry(self)
     48     def _setup_geometry(self):
     49         mylog.debug("Initializing Particle Geometry Handler.")
---> 50         self._initialize_particle_handler()
     51 
     52     def get_smallest_dx(self):

//anaconda3/lib/python3.7/site-packages/yt/geometry/particle_geometry_handler.py in _initialize_particle_handler(self)
    102                 ds.domain_left_edge, ds.domain_right_edge,
    103                 [N, N, N], len(self.data_files))
--> 104         self._initialize_indices()
    105         self.oct_handler.finalize()
    106         self.max_level = self.oct_handler.max_level

//anaconda3/lib/python3.7/site-packages/yt/geometry/particle_geometry_handler.py in _initialize_indices(self)
    132                 npart = data_file.total_particles[index_ptype]
    133             morton[ind:ind + npart] = \
--> 134                 self.io._initialize_index(data_file, self.regions)
    135             ind += npart
    136         morton.sort()

//anaconda3/lib/python3.7/site-packages/yt/frontends/gadget/io.py in _initialize_index(self, data_file, regions)
    351             count = sum(data_file.total_particles.values())
    352             return self._get_morton_from_position(
--> 353                 data_file, count, 0, regions, DLE, DRE)
    354         else:
    355             idpos = self._ptypes.index(self.index_ptype)

//anaconda3/lib/python3.7/site-packages/yt/frontends/gadget/io.py in _get_morton_from_position(self, data_file, count, offset_count, regions, DLE, DRE)
    330     def _get_morton_from_position(self, data_file, count, offset_count,
    331                                   regions, DLE, DRE):
--> 332         with open(data_file.filename, "rb") as f:
    333             # We add on an additionally 4 for the first record.
    334             f.seek(data_file._position_offset + 4 + offset_count * 12)

FileNotFoundError: [Errno 2] No such file or directory: '/Users/joost/Documents/research/snap_000.0'

```

**Expected outcome**

<!--A description of the expected outcome from the code snippet-->
Supposed to say catalogue generated.
<!--If this used to work in an earlier version of yt, please note the
version it used to work on-->

**Version Information**
<!--Please specify your platform and versions of the relevant libraries you are
using:-->
  * Operating System: OSX
  * Python Version: 3.7.3
  * yt version: 3.6
  * Other Libraries (if applicable): 

<!--Please tell us how you installed yt and python e.g., from source,
pip, conda. If you installed from conda, please specify which channel you used
if not the default-->
I installed from pip

diff --git a/.github/workflows/codetour-check.yaml b/.github/workflows/codetour-check.yaml
index 855126b83..583326683 100644
--- a/.github/workflows/codetour-check.yaml
+++ b/.github/workflows/codetour-check.yaml
@@ -12,6 +12,6 @@ jobs:
               uses: actions/checkout@v2
 
             - name: 'Watch CodeTour changes'
-              uses: pozil/codetour-watch@v1.3.0
+              uses: pozil/codetour-watch@v1.5.0
               with:
                   repo-token: ${{ secrets.GITHUB_TOKEN }}
diff --git a/yt/frontends/gadget/data_structures.py b/yt/frontends/gadget/data_structures.py
index 764d5b024..ce0c72ec2 100644
--- a/yt/frontends/gadget/data_structures.py
+++ b/yt/frontends/gadget/data_structures.py
@@ -157,7 +157,7 @@ class GadgetBinaryHeader:
         for filename in [self.filename, self.filename + ".0"]:
             if os.path.exists(filename):
                 return open(filename, "rb")
-        raise RuntimeError(f"Snapshot file {self.filename} does not exist.")
+        raise FileNotFoundError(f"Snapshot file {self.filename} does not exist.")
 
     def validate(self):
         """Validate data integrity."""
diff --git a/yt/geometry/particle_geometry_handler.py b/yt/geometry/particle_geometry_handler.py
index 5fecbd5de..517a76f8d 100644
--- a/yt/geometry/particle_geometry_handler.py
+++ b/yt/geometry/particle_geometry_handler.py
@@ -89,7 +89,13 @@ class ParticleIndex(Index):
             else:
                 end = None
             while True:
-                df = cls(self.dataset, self.io, template % {"num": i}, fi, (start, end))
+                try:
+                    _filename = template % {"num": i}
+                    df = cls(self.dataset, self.io, _filename, fi, (start, end))
+                except FileNotFoundError:
+                    mylog.warning(
+                        "Failed to load '%s' (missing file or directory)", _filename
+                    )
                 if max(df.total_particles.values()) == 0:
                     break
                 fi += 1
['yt/geometry/particle_geometry_handler.py', 'yt/frontends/gadget/data_structures.py', '.github/workflows/codetour-check.yaml']