virt: github reports cache self-cleaning when possible

When an exception is thrown upon reading from cache,
attempt to remove the related entries being operated on.
An exception will still be thrown, however the cache
will not be corrupted with bad entries.

Also, ran into RH BZ 954156 when running cache_populate.py
as it loads commit authors for issue #362:

bsddb.db.DBPageNotFoundError: (-30986, 'DB_PAGE_NOTFOUND: Requested page
not found')

RH BZ: 954156

Signed-off-by: Chris Evich <cevich@redhat.com>