Improve type checking error reporting

[od0x0]

The error checking was implemented as a pile of asserts while I was hacking things together. This means that most errors will lack enough context to understand the offending issue and debugging an issue in a schedule requires extensive knowledge of the compiler.

I expect that I'll be the one to write most examples in the short term, so this might be something we can defer until after a paper, but there's a limit to how long that will be a good idea.