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

[kripken]

Discussed in #17266

^{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