Gracefully handle errors in syntax highlighting by leaving the block unformatted

A bug in Rouge caused an Exception:

```undefined method `sub' for :Literal:Symbol```

That caused https://gitlab.com/embeddable-common-lisp/ecl/issues/156 to hit Error 500 and fail to display. If a failure occurs, just render the text as preformatted.

Closes #2433

See merge request !1274