Cryptic `sig is undefined` error in reportUndefinedSymbols() if function signature not available

^{Originally posted by **johnwason** June 18, 2022}
Currently if a function signature is not defined, `reportUndefinedSymbols()` will simple raise an error `sig is undefined`, which is not at all helpful. An error handler should be added to check if the value passed to `addFunction()` does not have `sig` defined, and report something more useful to the user. See #17264