checkMatrix return type refinement

[ye-luo]

I saw repetitive line CHECKED_ELSE(check.result) { FAIL(check.result_message); } in https://github.com/QMCPACK/qmcpack/pull/4732/files

The code can be simplified by returning bool and throw error in checkMatrix instead of {bool, string}

[PDoakORNL]

The string allows one to see per matrix element failures which I've used basically everytime I've needed this for debugging. Throws are slow so if you are just checking a matrix this is bad.

[ye-luo]

I'm not asking to throw immediately. Instead of return a string, you check the string and throw if it is not empty (indicating error). Throws are slow I don't think it is a concern.