Fix err bucket not found

[prarit]

fix(db): Ignore missing buckets during cleanup

The cleanup deletes all buckets after a build completes.  When building
with --only-update for a set of sources, some of the buckets in the
overall set are never created because the sources that populate them are
skipped.  Then cleanup fails with "bucket not found" even though the data
update completed.

Ignore bolt.ErrBucketNotFound.  This is safe to do since a bucket that
doesn't exist doesn't have to be cleaned up.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>