Add an error message for the case where a recruit does not match any recruit lists.

This should never happen, but if it does, it could be helpful to have the error
message distinguish this case from not having an eligible leader on a keep.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56182 75b3775d-3801-0410-9e12-e2e43121521d