Document how to return errors from actions

[viridia]

Currently the documentation makes no mention of how to return an error code from a createRouteAction or createServerAction. I assume it's one of either throw some error, or return a response object with an HTTP status code.

[viridia]

Some specific asks:

• Document the proper uses of FormError.
• Explain how to throw an exception that causes a specific HTTP error code to be returned (404, 401, etc.)