closebugs: don't overwrite specific failure
    
    If we hit a failure when trying to update a bug and we did get a
    specific HTTPError, but we also didn't touch the bug (which seems
    likely), we'll first set `failed_bugs[bug]` to the specific error
    from the HTTPError, but then overwrite it with the generic text
    "Unknown Bugzilla failure", which doesn't seem like a good idea.
    We should only set it to "Unknown Bugzilla failure" if we didn't
    get a specific HTTPError already.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>