yum-validator: avoid content overrides if possible, bail on fail

Adds logic to check if an override exists for a repo attribute and only
sets the new value via content override if an override already is in
place. Also added the `SubscriptionManagerError` exception so
`oo-admin-yum-validator` will bail with return code=2 if a call to
`subscription-manager` fails

Enterprise bug: 1166270
https://bugzilla.redhat.com/show_bug.cgi?id=1166270