Check PMIx Fence return status in MPI_Init

[artpol84]

One of the customers recently encountered the problem in the Slurm/PMIx environment.
For some unrelated reasons PMIx_Fence was cancelled by timeout by RM. However because OMPI doesn't checks the return status of the operation all processes proceeded further:

• modex: https://github.com/open-mpi/ompi/blob/v3.0.x/ompi/runtime/ompi_mpi_init.c#L663
• barrier: https://github.com/open-mpi/ompi/blob/v3.0.x/ompi/runtime/ompi_mpi_init.c#L840
  I think this is not a correct behavior and processes should take an error code path in this case.