Return non-200 status when ws cannot connect to db

If the database server isn't accepting more connections, a ws query returns an error

08006 DBI connect('dbname=musicbrainz_db;host=11.11.11.11;port=65401','musicbrainz',...) failed: ERROR: no more connections allowed (max_client_conn)

this error message is returned with a 200 status code, which means that some clients try and parse it as xml. This should be a 5xx status code instead, to indicate to clients that an error occurred