Fix mirror failover for instrepo

mirror_failover_obj is an additional callback object that happens when
there's a mirror failure. The default mirror_failure callback already
handles connection timeout and HTTP error 503 correctly (see _setupGrab
in yum/yumRepo.py).

After that, it calls mirror_failure_obj. In our case, that made it raise
the exception again, which makes us bail out of the download
prematurely.

Dropping this should fix that.