New Issue: Produce better error if doing GPU code generation and CHPL_COMM=none

18858, "stonea", "Produce better error if doing GPU code generation and CHPL_COMM=none", "2021-12-14T17:55:49Z"

As shown here: chpl__gpu.fatbin not generated · Issue #18850 · chapel-lang/chapel · GitHub GPU code generation does not work if CHPL_COMM is something other than none.

As of the release if you do this you'll get a confusing error message about a missing fatbin file at runtime. I believe on our working branch now you'll get a confusing error message about a missing fatbin file at compiletime (though I need to check).

Either way we should produce a better error or at least give the user a warning about it a compile time. Ultimately we plan to have this supported but I'm not convinced that'll happen before the 1.26 so I'd rather add the warning\error now before we forget.