Inform the user if no repositories are enabled when running certain actions

[Justinzobel]

If the user runs actions that would look at a repository and there are none enabled or none exist, let the user know.

Somehow recently my main and only repository, the stable Shannon repository, got disabled.

I just tried to search for something and eopkg sr showed no output at all. I was confused as I could see the package here in the GitHub repo.

Then I searched for just the letter a knowing it would show results, but it didn't. It took me a bit of confusion and troubleshooting to realise the repository wasn't enabled.

A quick check by eopkg before these commands run to confirm:

1. There are repositories
2. At least one of those repositories is enabled

This would have exposed the root cause of the issue.