Panic if the grammar verifier sees a token it doesn't recognize

To prevent the reference grammar from getting out of sync with the real
grammar, panic if RustLexer.tokens contains an unknown token in a
similar way that verify.rs panics if it encounters an unknown binary
operation token.

src/grammar/verify.rs

@@ -111,7 +111,7 @@ fn parse_token_list(file: &str) -> HashMap<String, token::Token> {
             "LIT_BINARY_RAW"    => token::Literal(token::BinaryRaw(Name(0), 0), None),
             "QUESTION"          => token::Question,
             "SHEBANG"           => token::Shebang(Name(0)),
-            _                   => continue,
+            _                   => panic!("Bad token str `{}`", val),
         };
 
         res.insert(num.to_string(), tok);