Informative error messages

[jonsterling]

I'm mostly meaning "informative for the developer" than for the user at this point---the problem is that in many parts of the code, we just ask whether something typechecks or is equal, and we don't remember the reason why it failed, so I get this inscrutable error that is impossible to debug.

I'd like to have some lightweight way to at least remember what went wrong. A sum type instead of booleans would be a start.