Hangs when rhombus lexer errors

[greghendershott]

I get the hangs particularly when the lexer errors. See racket/rhombus#573 for related info.

Originally posted by @samth in #726 (comment)

[greghendershott]

Although the ultimate, best fix will be for the lexer not to error, I'll look at handling lexer exceptions more smoothly than freezing.

(It might not be much smoother -- racket-hash-lang-mode depends on the lexer for nearly everything, from navigation to coloring. I might need to change the buffer's major mode back to plain old prog-mode or racket-mode. Which would still suck, just not as badly as freezing.

In any case my goal should be to reflect enough exception details up into an Emacs error message, so users understand and we know roughly what to do, where.)