Catch against max depth errors

Bison allows you to define a `YYMAXDEPTH` to [prevent memory exhaustion](https://www.gnu.org/software/bison/manual/html_node/Memory-Management.html) from excessive recursion.

I've used [this KaTeX bug](http://git.io/FlWHfw) as an arbitrary upper limit for what mtex2MML should be able to do. In practice, mtex2MML can do much more than the limit of 430 nestings, but it's probably best to keep this simple. 

/cc @arfon