Fix error handling in rpmhdrs_rpmdbv()

Letting GErrors pile up is wrong; either exit the function or clear the
GError (perhaps with a console warning).

In this case we tolerate missing database files (because users are free
to delete them), and build the checksum from whatever data is available.
Seems weird, but that's how I'm told it's meant to work.  Bail out on
any other type of error.