Async task tracebacks should be properly handled in case of error

[sentry]

EncodeError: Object of type 'traceback' is not JSON serializable
Sentry Issue: STUDIO-EA2

DatabaseError: Save with update_fields did not affect any rows.
(5 additional frame(s) were not displayed)
...
  File "contentcuration/models.py", line 1763, in save
    super(ContentNode, self).save(*args, **kwargs)
  File "mptt/models.py", line 1009, in save
    super(MPTTModel, self).save(*args, **kwargs)
  File "django/db/models/base.py", line 808, in save
    force_update=force_update, update_fields=update_fields)
  File "django/db/models/base.py", line 838, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "django/db/models/base.py", line 909, in _save_table
    raise DatabaseError("Save with update_fields did not affect any rows.")

TypeError: Object of type 'traceback' is not JSON serializable
(3 additional frame(s) were not displayed)
...
  File "__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "kombu/utils/json.py", line 59, in default
    return super(JSONEncoder, self).default(o)
  File "json/encoder.py", line 180, in default
    o.__class__.__name__)

EncodeError: Object of type 'traceback' is not JSON serializable
(16 additional frame(s) were not displayed)
...
  File "__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "kombu/utils/json.py", line 59, in default
    return super(JSONEncoder, self).default(o)
  File "json/encoder.py", line 180, in default
    o.__class__.__name__)

Task move_nodes_task[16d16ea4640948dba75408d0c7ecc28e] raised unexpected: EncodeError(TypeError("Object of type 'traceback' is not JSON serializable",),)