[packagekit] yum: detecting file conflicts and using a custom error

[Richard Hughes]

The YUM backend seems to report that file conflicts is an unknown type
of error. This sort of error is quite common when users mix repos (and
they do) so we probably need to explicitly catch this and emit a custom
error code.

There's a screenshot here:
https://bugzilla.redhat.com/attachment.cgi?id=300088

Thanks.

Richard.