partially-complete transfers prevent a successful upgrade
### Observed behavior

If Kolibri 0.6 is shut down while a transfer is in progress, running Kolibri 0.7 won't work

### Expected behavior

Upgrades should occur smoothly. Preferably the transfer can pick up where it left off, but if it needs to be canceled that's the next best solution.

### User-facing consequences

Kolibri is not usable after attempting to upgrade.

### Errors and logs

lots of this:

```
ERROR    Internal Server Error: /api/tasks/
Traceback (most recent call last):
  File "/Users/d/PythonEnvs/le2/lib/python2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/d/PythonEnvs/le2/lib/python2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Users/d/PythonEnvs/le2/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/Users/d/PythonEnvs/le2/lib/python2.7/site-packages/rest_framework/viewsets.py", line 87, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Users/d/PythonEnvs/le2/lib/python2.7/site-packages/rest_framework/views.py", line 466, in dispatch
    response = self.handle_exception(exc)
  File "/Users/d/PythonEnvs/le2/lib/python2.7/site-packages/rest_framework/views.py", line 463, in dispatch
    response = handler(request, *args, **kwargs)
  File "kolibri/tasks/api.py", line 42, in list
    jobs_response = [_job_to_response(j) for j in get_client().all_jobs()]
  File "kolibri/tasks/api.py", line 427, in _job_to_response
    "type": job.extra_metadata.get("type"),
AttributeError: 'Job' object has no attribute 'extra_metadata'
```

### Steps to reproduce

* start up 0.6
* start a long-running import
* shut down 0.6 before import completes
* start up 0.7


### Context

* Kolibri version: 0.6 -> 0.7
* Operating system: OSX

diff --git a/kolibri/tasks/api.py b/kolibri/tasks/api.py
index affaac1a57..e72f56a62d 100644
--- a/kolibri/tasks/api.py
+++ b/kolibri/tasks/api.py
@@ -424,8 +424,8 @@ def _job_to_response(job):
         }
     else:
         return {
-            "type": job.extra_metadata.get("type"),
-            "started_by": job.extra_metadata.get("started_by"),
+            "type": getattr(job, "extra_metadata", {}).get("type"),
+            "started_by": getattr(job, "extra_metadata", {}).get("started_by"),
             "status": job.state,
             "exception": str(job.exception),
             "traceback": str(job.traceback),
['kolibri/tasks/api.py']