fix: Don't bail on invalid params

[benbrandt]

• Restructure control flow
• cleanup error
• Cleanup and add test

[nikomatsakis]

I think this PR looks good. I like the tests. I'd be happy to merge it

However, I feel like the overall approach is kind of fragile. I feel like it'd be nice to just put ? in handlers and not have to think too hard. I wonder if what we ought to do instead is to change the way that we deal with errors from handlers (and maybe tasks and other user-supplied code) so that, instead of propagating them, we actually just swallow them and report them back to the other side? Kind of like how a web server will catch an unhandled exception and recover?

We could have some sentinel type for a "fatal error" that truly propagates out.

[benbrandt]

Closing in favor of #114