WIP: ProcError()

This is a dumb hack to make debugging easier, by giving you a place to set a breakpoint to catch protocol error generation. It's not close to complete but it should be pretty mechanical to finish the core bits at least.

On its own this isn't terribly useful, but once you have it you can pass as much information down to ProcError() as you want. Maybe you want to emit a generic event pointing to the file/line number for each error, maybe you want to log some categories of error, etc. We can also mark the error functions as cold so the compiler can pessimize those branches.