Implement Response.error()

[jasnell]

To this point we have not implemented Response.error() with the explanation:

  // TODO(conform): implementation is missing; two approaches where tested:
  //  - returning a HTTP 5xx response but that doesn't match the spec and we didn't
  //    find it useful.
  //  - throwing/propagating a DISCONNECTED kj::Exception to actually disconnect the
  //    client. However, we were concerned about possible side-effects and incorrect
  //    error reporting.

WinterTC will be recommending that conformant implementations support Response.error(). We should implement it.

When a request is answered with an error response, the return back to the client will be a 500 error and a runtime error will be reported on the server-side. Whenever an error response is used anywhere we accept a Response an error will be thrown.

Response.error() use is surprisingly common.

Necessary for:

• WinterTC Conformance
• Node.js Conformance