Add `errorClass` or similar to specify HTML class to be used for erroneous segments

At the moment, there is very limited control over formatting for invalid LaTeX. We can pass `errorColor` (with `throwOnError: false`), or do manual error handling. It would be useful to have something like `errorClass` to add a custom class for formatting to any erroneous commands, etc. in the LaTeX, so we can control things like font, size, etc.