Display specific error messages when parts of the courseware break, rather than returning a 500 indicating that the whole site is down

Fix missing `global_context` problem

This problem was introduced in 232e758c
and caused the check answer button to not functioning.

Multiple Choice support. Outstanding issues: 
* Cleanup of the way names are handled
* Figure out why $.ajax needs a stringify
* I'd prefer to keep both input types and response types as classes. This is necessary for richer, AJAXy interactions.