XDCR - give better error messages under migration mode

With MB-42907 implemented, it is possible for user to enter more than 1 rule under the UI for the MB-42907 use case for a special case of migration where it is a single explicit mapping from the source bucket's default collection to a particular target collection.

XDCR internally needs to produce a better error than an out-of-context "Key-only regexp is not supported".

This is marked critical because a confusing error will cause usability issues and potential un-necessary customer support cases.