db-move: Clarify error message when the specified repo isn't correct for the current dbscripts instance
It's not entirely clear what a "configured repository" is, but hopefully saying that the given repo isn't a "valid repository" is clearer.
In typical use this will result in an error like this:
==> ERROR: extra is not a valid repository for /community/db-moveor this:
==> ERROR: community-testing is not a valid repository for /packages/db-move