get/import: improve error message on no default remote
Since we allow importing/getting files that are not cached this error does not make much sense:

```
failed to import 'model.pkl' from '../test-import/'. - config file error: no remote specified. Setup default remote with
```

in a lot of cases.

diff --git a/dvc/exceptions.py b/dvc/exceptions.py
index 07a0ce806..291606614 100644
--- a/dvc/exceptions.py
+++ b/dvc/exceptions.py
@@ -328,3 +328,13 @@ class CheckoutError(DvcException):
 
 class CollectCacheError(DvcException):
     pass
+
+
+class RemoteNotSpecifiedInExternalRepoError(DvcException):
+    def __init__(self, url, cause=None):
+        super(RemoteNotSpecifiedInExternalRepoError, self).__init__(
+            "No DVC remote is specified in the target repository '{}'".format(
+                url
+            ),
+            cause=cause,
+        )
diff --git a/dvc/external_repo.py b/dvc/external_repo.py
index ccdadf6e6..874240bc7 100644
--- a/dvc/external_repo.py
+++ b/dvc/external_repo.py
@@ -7,6 +7,8 @@ from distutils.dir_util import copy_tree
 
 from funcy import retry
 
+from dvc.config import NoRemoteError
+from dvc.exceptions import RemoteNotSpecifiedInExternalRepoError
 from dvc.utils import remove
 
 
@@ -19,7 +21,10 @@ def external_repo(url=None, rev=None, rev_lock=None, cache_dir=None):
 
     path = _external_repo(url=url, rev=rev_lock or rev, cache_dir=cache_dir)
     repo = Repo(path)
-    yield repo
+    try:
+        yield repo
+    except NoRemoteError as exc:
+        raise RemoteNotSpecifiedInExternalRepoError(url, cause=exc)
     repo.close()
 
 
['dvc/external_repo.py', 'dvc/exceptions.py']