mpi4py: overly-verbose and misleading output from MPI_Group_from_session_pset()

[dalcinl]

Trying to create a group from a nonexistent session pset fails with an overly verbose and misleading output message. IMHO, I would argue that nothing should be printed to stdout/stderr, at least if MPI_ERRORS_RETURN is the active error handler. While I would be willing to accept the first part of this extra output, the second part mentions MPI_INIT and is quite misleading.

>>> from mpi4py import MPI
>>> s = MPI.Session.Init()
>>> s.Create_group("dummy")

--------------------------------------------------------------------------
Your application has invoked an MPI function that is not supported in
this environment.

  MPI function: MPI_Group_from_session_pset
  Reason:       PMIx server unreachable
--------------------------------------------------------------------------
--------------------------------------------------------------------------
It looks like MPI_INIT failed for some reason; your parallel process is
likely to abort.  There are many reasons that a parallel process can
fail during MPI_INIT; some of which are due to configuration or environment
problems.  This failure appears to be an internal failure; here's some
additional information (which may only be relevant to an Open MPI
developer):

  PMIx_Query_info() failed
  --> Returned "(null)" (52) instead of "Success" (0)
--------------------------------------------------------------------------
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    s.Create_group("dummy")
    ~~~~~~~~~~~~~~^^^^^^^^^
  File "src/mpi4py/MPI.src/Session.pyx", line 119, in mpi4py.MPI.Session.Create_group
    CHKERR( MPI_Group_from_session_pset(
mpi4py.MPI.Exception: MPI_ERR_UNSUPPORTED_OPERATION: operation not supported

The first part of the error is due to not using mpiexec, however the second part shows up even if using mpiexec. In this case I see no justification for this extra misleading output.

$ mpiexec -n 1 python -c 'from mpi4py import MPI; MPI.Session.Init().Create_group("dummy")'
--------------------------------------------------------------------------
It looks like MPI_INIT failed for some reason; your parallel process is
likely to abort.  There are many reasons that a parallel process can
fail during MPI_INIT; some of which are due to configuration or environment
problems.  This failure appears to be an internal failure; here's some
additional information (which may only be relevant to an Open MPI
developer):

  PMIx_Query_info() failed
  --> Returned "(null)" (13) instead of "Success" (0)
--------------------------------------------------------------------------
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    from mpi4py import MPI; MPI.Session.Init().Create_group("dummy")
                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "src/mpi4py/MPI.src/Session.pyx", line 119, in mpi4py.MPI.Session.Create_group
    CHKERR( MPI_Group_from_session_pset(
mpi4py.MPI.Exception: MPI_ERR_ARG: invalid argument of some other kind

This is very low hanging fruit and definitely not a serious issue, but maybe something to consider improving if that part of the code is ever refactored or worked on.

cc @hppritcha