Have SMPI fail on MPI_ERR_* in MC mode
The standard says that upon error, implementations should call the
current MPI error handler, which is MPI_ERRORS_ARE_FATAL by default
but could be changed to MPI_ERRORS_RETURN on need.
Since we don't implement MPI_Comm_set_errhandler() to switch between
modes, the simulation mode of SMPI is only issuing a warning on
errors (which is similar to MPI_ERRORS_RETURN).
This commit adds a MC_assert() stating that every MPI call succeed.
This will lead to a property failure (visible only in MC mode) when a
MPI_ERR_* is issued by the implementation (which is somehow similar to
MPI_ERRORS_ARE_FATAL).
