Fixes #4554: Move r28584 and r28618 to the v1.8 branch (MPI_T stuff)

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

There were conflicts when applying these r numbers to the v1.8 branch
because the missing RMA error codes patch was applied first (i.e., out
of order compared to actual commit order on the trunk).  But the
conflicts were trivial to fix, so I just took the liberty of fixing
them as part of the GK CMR move.

r28584 [[BR]]
Add useful messages for MPI_T error codes

r28618 [[BR]]
Add destructors for MPI_T error codes