Make error reporting not stupid.

[AiredaleDev]

Currently error handling is to tight and annoying to use. I consistently find myself using ErrorType::OneOff in most places because a hand-written message from me is better 99% of the time. I also need to change the way error handling works so I can easily accumulate them for the user.

Perhaps I should also only report many errors when parsing (or later) fails -- the signal to noise ratio on lexing errors that follow the first one is likely poor.

[AiredaleDev]

The ergonomics on constructing errors is improved, but I still need a way to accumulate them. Luckily I have a good excuse to do this alongside type-checking, since anything that parses should be structured enough s.t. I can justify reporting errors all over the program. Not quite sure how I'd report multiple parsing errors in a sane way, or maybe it's easier than I think it is.

EDIT: I'm very silly -- I should just report them exactly where they happen and just pass up some signal that says "stop compilation pls" if anything fatal happens.