Don't suppress errors in run-cover.

Seriously. Why did I think that was a good idea?

Change-Id: I161a97d36592f271c7f2b1291acfd87b12199118
Reviewed-on: https://review.openstack.org/30524
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins