Warn on reserved words

[Akron]

When Antlr stumbles upon reserved words in wrong contexts, it will stop parsing, pointing to the first failing symbol. It would be nice to throw an error message giving a hint, that the user tried to use a reserved term. A first start may be, taking the token inventory (or hardcoded) in a regex following a word boundary (at least in PQ+), and in case that matches, responding with "Maybe use of a reserved token".

Reported example in PQ+: split.