yum-validator: Fix crash on non-RHSM systems

The RHSM logic didn't account for unregistered systems, so the missing
key/cert for RHSM registration would raise an uncaught exception.

Added code to trap this exception and raise the more meaningful
`SubscriptionManagerNotRegisteredError`, and added logic in
`oo-admin-yum-validator` to properly handle this exception.

Enterprise bug: 1172444
https://bugzilla.redhat.com/show_bug.cgi?id=1172444
Merge pull request #509 from dobbymoodge/BZ1172444

yum-validator: Fix crash on non-RHSM systems