Improve message and handling of internal compiler errors

[WardBrian]

We may want to install a top-level exception handler which catches all exceptions, manually prints them and the backtrace, and then outputs the message Fatal error: this should never happen. Please file a bug.... If we did this, we wouldn't even need the Common.FatalError module, just using raise_s everywhere we need would work.

I believe this also would improve the javascript parser, since currently there internal compiler errors manifest as exceptions rather than populating the .error field of the return object