HTTP 500 errors on incorrect bugzilla credential entry
    
    Caused by a change in the interface in python-bugzilla:
    https://git.fedorahosted.org/cgit/python-bugzilla.git/commit/?id=2e36c4092bf644d41bb5c80322a10512458c0bdf
    
    Also renames BugzillaError exception since its names collides with
    python-bugzilla's exception of the same name and creates confusion.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D249