Catch sqlalchemy's DatabaseError in fetch and gather callback

I sometimes see "connection timed out" message which are reported as
sqlalchemy.exc.DatabaseError, so by catching the latter exception, it'd
avoid the harvester to be stuck in "limbo" state.

As DatabaseError is a super-class of OperationalError, the latter would
still be catched.

ckanext/harvest/queue.py

@@ -274,9 +274,10 @@ def gather_callback(channel, method, header, body):
 
  try:
  job = HarvestJob.get(id)
- except sqlalchemy.exc.OperationalError:
+ except sqlalchemy.exc.DatabaseError:
  # Occasionally we see: sqlalchemy.exc.OperationalError
  # "SSL connection has been closed unexpectedly"
+ # or DatabaseError "connection timed out"
  log.exception('Connection Error during gather of job %s', id)
  # By not sending the ack, it will be retried later.
  # Try to clear the issue with a remove.
@@ -374,9 +375,10 @@ def fetch_callback(channel, method, header, body):
 
  try:
  obj = HarvestObject.get(id)
- except sqlalchemy.exc.OperationalError:
+ except sqlalchemy.exc.DatabaseError:
  # Occasionally we see: sqlalchemy.exc.OperationalError
  # "SSL connection has been closed unexpectedly"
+ # or DatabaseError "connection timed out"
  log.exception('Connection Error during fetch of job %s', id)
  # By not sending the ack, it will be retried later.
  # Try to clear the issue with a remove.