Try to catch / workaround 5215 (#5216)

Fixes #5215

It's not really clear how the error can be obtained, but this PR adds a
workaround for the problem. There are 2 possible ways to get the
`message` parameter - from an error and from the operation status.

- Our status messages are always represented by a string, no any other
values is assigned.

- rq is trickier here - it receives rq data and [decodes
it](https://github.com/rq/rq/blob/master/rq/job.py#L603-L609) if there
is an error, but the operations can leave None as the `exc_info` value.
Maybe [this issue](https://github.com/rq/rq/issues/1633) is relevant
here.

cvat/apps/engine/views.py

@@ -1221,10 +1221,14 @@ class TaskViewSet(viewsets.GenericViewSet, mixins.ListModelMixin,
         elif job.is_queued:
             response = { "state": "Queued" }
         elif job.is_failed:
-            response = { "state": "Failed", "message": job.exc_info }
+            # FIXME: It seems that in some cases exc_info can be None.
+            # It's not really clear how it is possible, but it can
+            # lead to an error in serializing the response
+            # https://github.com/opencv/cvat/issues/5215
+            response = { "state": "Failed", "message": job.exc_info or "Unknown error" }
         else:
             response = { "state": "Started" }
-            if 'status' in job.meta:
+            if job.meta.get('status'):
                 response['message'] = job.meta['status']
             response['progress'] = job.meta.get('task_progress', 0.)