Fix internal error with selection "resname help".

Fix internal error with selection "resname help".

- Always recognize "help" as a help request token, which then causes a
  syntax error in invalid context instead of an internal error in the
  tokenizer symbol table lookup.
- Change the internal error in question to use an exception for error
  reporting to make it possible to easily check it in a unit test.

Change-Id: I4a4057840b47a818cc840654945e84ec536e35b0