Modified a few error messages in tests/mdist.R.
Merge branch 'master' of github.com:markmfredrickson/optmatch

Conflicts:
	tests/mdist.R
I had made minor edits some time ago to tests/mdist.R.  This was my attempt to merge them back in, without in any real way changing the tests file.  (The only intended change is that a few errors, when thrown, should be a bit more informative.  But these errors shouldn't be thrown except when something has been broken.)
Merge branch 'master' of github.com:markmfredrickson/optmatch
Fixing (I hope) error message adjustments in tests/mdist.R.