I'm going to leave this PR open until I go through the docs and rewrite / add content for error sets, and merge it in manually.

Don't worry, feel free to close if you're rewriting the docs anyway. Also happy to rebase when you're done.

I added a little bit to the docs on errors. Here's what I ended up writing:

The Global Error Set

error refers to the global error set. This is the error set that contains all errors in the entire compilation unit. It is a superset of all other error sets and a subset of none of them.

You can implicitly cast any error set to the global one, and you can explicitly cast an error of global error set to a non-global one. This inserts a language-level assert to make sure the error value is in fact in the destination error set.

The global error set should generally be avoided when possible, because it prevents the compiler from knowing what errors are possible at compile-time. Knowing the error set at compile-time is better for generated documentationt and for helpful error messages such as forgetting a possible error value in a switch.