Don't error out for over 500 results

We can legitimately get more than 500 when updating a large number of packages
and then showing the offline update results.

This commit drops the failure path for >500 results; for cases where
performance is critical (search), the results are already truncated in
gs_plugin_loader_job_sorted_truncation_again() and for other cases we
can just keep on going.

https://bugzilla.redhat.com/show_bug.cgi?id=1496489