Raise explicit errors gracefully instead of returning SystemError code 1

[Luispain]

Dear all,

This ticket is to request for having explicit error handling, for allowing Python interrupt gracefully with relevant exception types and error message, instead of interrupting the simulation in a hard way such as in:

E             File "/stck/cassiope/git/Cassiopee/Dist/bin/ld/lib/python3.8/site-packages/FastS/PyTree.py", line 2073, in display_temporal_criteria
E               import sys; sys.exit(1) # return 1 to shell
E           SystemExit: 1

Where we have:

    if stopAtNan:
        finite = Cmpi.isFinite(t, var='centers:Density')
        if not finite:
            import sys; sys.exit(1) # return 1 to shell

At least I would rather prefer something like:

    if stopAtNan:
        finite = Cmpi.isFinite(t, var='centers:Density')
        if not finite:
            raise ValueError("NaN where detected at Density field")

Which explicitly tells what fails, and it is easier to handle the exception through try/except blocks, or other exception handling mechanisms.

It would be easier because we can automatically know what actually caused the solver to crash, because using try/except handling for SystemError with value 1 does not tell much of what caused the problem.

Best regards

Luis