update_sync: fix Bodhi queries when F33M release doesn't exist
Not all releases exist all the time. Currently F33 and F33C releases exist, but
F33M doesn't. With the existing code, that completely breaks all F33 update
querying. So instead, retreive the list of releases known to Bodhi and only
skip those which are not known, instead of all-or-nothing approach. F33 updates
now correctly show up with this patch.
Merges: https://pagure.io/fedora-qa/blockerbugs/pull-request/128