fix: wording for message related to OPAMDONOTCOPYFILES
Merge pull request #1246 from pw374/master

fix: wording for message related to OPAMDONOTCOPYFILES
Ensure conflicts are never silent

even when we can't get any explanation for some reason ;
also add explanation on some constraint failures.
Better causes on removed packages
Fixed handling of upstream-removed packages at the solver level
Cleanup orphan package handling in OpamClient