[hsjs] Avoid calling error handlers twice in the same request

There are some cases where through composition of components, the JS server generator can try to respond twice in error conditions, for example:

- During body deserialization, client data is malformed, so the body deserializer responds with a 400.
- Body deserialization rejects, causing the route dispatch function to throw.
- The router catches the error, and tries to respond with a 5xx internal server error.
- The Node server runtime sees the response has already ended, and throws an error from the onInternalError handler.
- The router is out of `catch`es when the error-catching code itself errors, so the error bubbles up.
- The process gets an uncaught error and dies.

I think there are two things to do here:

1. Even if the error-handling code throws, the process shouldn't be killed.
2. Make all the error handlers run-once in a group. I.e. if you call `onInternalError` after `onInvalidRequest`, log a warning saying the `onInternalError` is ignored and don't try to respond again.