Don't log an exception we're ignoring like we didn't ignore it
    
    As the comment says, all we really want to do here is catch an
    expected exception and ignore it. But using `log.exception` is
    an odd thing to do in this case. `log.exception` logs in a way
    that looks *exactly like* the exception was not caught and the
    command crashed:
    
    [adamw@adam blockerbugs (develop)]$ python3 run_cli.py -f sync
    ...
    update_sync  2019-05-01 17:02:04 INFO    found 7 bugs in f31 release
    update_sync  2019-05-01 17:02:04 ERROR   Invalid releases specified: f31
    Traceback (most recent call last):
      File "/home/adamw/local/blockerbugs/blockerbugs/util/update_sync.py", line 122, in search_updates
        result = self.bodhi_interface.query(**queries_data)
      File "/usr/lib/python3.7/site-packages/fedora/client/bodhi.py", line 118, in wrapper
        raise BodhiClientException(problems)
    fedora.client.bodhi.BodhiClientException: Invalid releases specified: f31
    [adamw@adam blockerbugs (develop)]$
    
    This is highly confusing, and I've now *twice* run into this,
    thought the sync was crashing, and spent half an hour wondering
    why the exception wasn't ignored as expected before I realized
    it *was* ignored as expected but this output was fooling me.
    So let's not log the exception as if we weren't ignoring it,
    let's instead log something that looks much less scary and makes
    it clearer what's going on.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>