Bugzilla.getExternalBugTrackerToUse() doesn't handle some non LP-Plugin Bugzillas correctly

[Graham Binns]

We're seeing a lot of errors from Bugzilla.getExternalBugTrackerToUse(), raised when attempting to call Launchpad.plugin_version() on the remote server. This is because we currently re-raise all ProtocolErrors that occur in getExternalBugTrackerToUse() whose error code isn't 404. Unfortunately, different Bugzillas seem to handle the calling of non-existent methods differently. Some (version 3.2, as far as I can tell) raise an XML-RPC Fault, others return an HTTP 500 error.

We already handle any raised Faults correctly; we should handle the HTTP 500s correctly, too.

[Graham Binns (gmb)]

Landed in r6657.