extras/jac3d: Improve command-line behavior

The new upcxx-extras/jac3d example has a complicated command-line interface that must be used correctly for anything to work. However the built program currently gives you very little help in getting it right.

Example:

$ upcxx-run -N 1 -n 4 ./jac3d 
ERROR # of processors (4) does not equal px * py * pz (1).
$ upcxx-run -N 1 -n 4 ./jac3d -h
Segmentation fault
$ upcxx-run -N 1 -n 4 ./jac3d --help
Segmentation fault

This issue is to request better output when command-line processing fails. At the least the argument validation should output the same usage info from the README. There should be no command-line input that leads to an opaque seg fault.

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