Report back errors in GetNumChildren() (#84265)

This is a proof-of-concept patch that illustrates how to use the
Expected return values to surface rich error messages all the way up
to the ValueObjectPrinter.

This is the final patch in the series that includes
https://github.com/llvm/llvm-project/pull/83501 and
https://github.com/llvm/llvm-project/pull/84219