Some tweaks to unit_type_data::set_config().

Most notably, this now checks to see if the insertion was successful and logs an error
if it failed (meaning two [unit_type]s had the same id).


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56093 75b3775d-3801-0410-9e12-e2e43121521d