New exception for mpi4py import

[opotowsky]

What is the change? Why is it being made?

ARMI has a try/except for the from mpi4py import MPI that has historically worked for us downstream. The exception, ImportError, has captured all our needs up until now.

Now (mpi4py 4.1.1 upgrade to go with the pkl5 change), that import has a different exception: RuntimeError (RuntimeError: cannot load MPI library). We are failing upon all invocations of armi as a result, unless we have an OMPI module loaded.

SCR Information

Change Type: features

One-Sentence Rationale: Upgrading mpi4py caused a new error; this update handles that situation.

One-line Impact on Requirements: NA

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.
• The documentation is still up-to-date in the doc folder.
• The code style follows good practices.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

@opotowsky Approved. And you have Merge power.