abrtd: mark unprocessed dump directories as not-reportable

If count item is missing in dump directory at abrtd start, then abrtd
considers that dump directory as unprocessed.

Closes #562

v2: don't update not-reportable if it already exists, better messages

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>