Implement error handling and the result type

[jackyzha0]

We'd like to have better ergonomics around when requests can fail.

• Adds the Result<T, E> type. RPC type signature changed from Promise<Output> to Promise<Result<Output, ErrorUnion>> where ErrorUnion is a union of all the declared error types. You can then switch on result.ok to get the Ok or Err cases with appropriate type narrowing.
• Adds the errors field to a procedure declaration.
• Returns from a procedure must be a result type. We introduce Ok(payload) and Err(payload) to simplify result construction. Again, both result arms are statically checked so you can't return messages or errors you don't declare in the output and errors fields respectively.
• Any uncaught exceptions in a handler are caught and wrapped in a result type that matches RiverUncaughtSchema (defined in router/result.ts)
• Bumps the package version to 0.5.0

[Monkatraz]

looks good to me. Jacky already answered some questions I had in Slack.