When Picard received an error 502 from acoustid.org, it should report that to the user.

If the 502 is on submit, the submit button should also remain enabled.

Example traceback/log entry on a 502:

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/picard/webservice.py", line 223, in _process_reply
handler(xml_handler.document, reply, error)
File "/usr/lib/python2.7/dist-packages/picard/acoustid.py", line 97, in _on_lookup_finished
results = document.response[0].results[0].children.get('result')
File "/usr/lib/python2.7/dist-packages/picard/webservice.py", line 76, in _getattr_
raise AttributeError, name
AttributeError: response
E: 139679397885696 16:33:33 Network request error for http://api.acoustid.org:80/v2/lookup: Error downloading http://api.acoustid.org:80/v2/lookup - server replied: Bad Gateway (QT code 301, HTTP code 502)