Make lexer tooling message more generic

Make lexer tooling message more generic

We don't actually probe for javac in all circumstances, so if you have
javac installed, but don't have antlr4 installed, and you're on Mac OS
X, then you'll get a message that javac is missing, even though that's
wrong.

To fix this, let's just be a bit more generic in the message, so that
it's the same no matter what part of the lexer tests you're missing.

cc
https://www.reddit.com/r/rust/comments/3m199d/running_make_check_on_the_source_code_says_javac/