add basic error handler functionality

[simonbyrne]

Fixes #453.

[simonbyrne]

Note that mpi4py sets MPI_ERRORS_RETURN on the default communicators:
https://mpi4py.readthedocs.io/en/stable/overview.html#error-handling
should we do the same?

[simonbyrne]

@kpamnany @vchuravy any thoughts on this?

[kpamnany]

This looks sensible. However, I've never used this capability, so don't really have any useful input.

[vchuravy]

So this will no longer kill the process on error? That seems sensible, altough you could still end up in a deadlock.

[simonbyrne]

So this will no longer kill the process on error? That seems sensible, altough you could still end up in a deadlock.

Yes, it will throw an MPIError instead.