Start: Allow throwing custom errors with non-500 status from server functions

[frenzzy]

Description:
When using @tanstack/start server functions, any thrown error except redirects/notFound is always turned into an HTTP 500, even if you previously called setResponseStatus(code). Conversely, returning an Error instance respects the status but serializes to an empty payload. This makes it impossible to send rich error details (e.g. validation failures) with the proper HTTP status.

Reproduction:

1. Wrap your RPC call in a middleware that does:

   setResponseStatus(400);
   throw new FormError('Invalid input', { errors: { field: ['Required'] } });

2. Observe on the client:

   • Status is 500, not 400.
   • JSON body contains generic { message, stack }, not the errors object.

3. Change to returning instead of throwing:

   setResponseStatus(400);
   return new FormError('Invalid input') as never;

Observe:

• Status is correctly 400, but response body is { result: { $undefined: 0 }, error: { $undefined: 0 }, … }.

Expected behavior:

• Ability to throw an error (or use a special helper) so that:
  1. The HTTP status matches what was set.
  2. The response body serializes the error’s properties (name, message, errors, etc.) in a stable format.

Possible solutions:

• Respect setResponseStatus(code) on thrown errors (not just on Response instances).
• Introduce a helper like throw errorWithStatus(payload, status) analogous to json()/redirect()/notFound().
• Allow extending a base error class (e.g. HttpError) with a status property that the runtime will honor.

Context:

• Using @tanstack/react-start + @tanstack/react-router (v1.128.3).
• Custom middleware detects gRPC failures and calls setResponseStatus(code) before throwing a FormError.
• Redirects and notFound() already work as expected.

Thanks! 🙏