Fixes #4509: RMA error names

---svn-pre-commit-ignore-below---

r31366 [[BR]]
mpi.h.in: fix comment to match code

Thanks to Lisandro Dalcin for noticing.

'''GK NOTE:''' Only used part of r28225 [[BR]]
Ensure that MPI_LASTUSEDCODE is always >= MPI_ERR_LASTCODE.  Thanks to
Dimitry Dontsov for reporting the issue and providing a patch.