Runcible: Overriding the default config error message

This goes along with Katello/runcible#64 and the goal was to show a message
where developers would realize they have to log in a user in order to call
Runcible methods. Previously we were just seeing '@@config undefined' when
calling stuff like "Repository.last.packages" from rails console.
Merge pull request #2332 from daviddavis/runcible-config-error

Runcible: Overriding the default config error message