Generate ssdk error tests

[JordonPhillips]

This updates the http protocol test generator to include error tests. A generated test looks like:

it("RestJsonInvalidGreetingError:ServerErrorResponse", async () => {
  class TestService implements Partial<RestJsonService> {
    GreetingWithErrors(input: any, request: HttpRequest): GreetingWithErrorsServerOutput {
      const response = {
        Message: "Hi",
      } as any;
      const error: InvalidGreeting = {
        ...response,
        name: "InvalidGreeting",
        $fault: "client",
        $metadata: {},
      };
      throw error;
    }
  }
  const service: any = new TestService();
  const testMux = new httpbinding.HttpBindingMux<"RestJson", keyof RestJsonService>([
    new httpbinding.UriSpec<"RestJson", "GreetingWithErrors">("POST", [], [], {
      service: "RestJson",
      operation: "GreetingWithErrors",
    }),
  ]);
  class TestSerializer extends GreetingWithErrorsSerializer {
    deserialize = (output: any, context: any): Promise<any> => {
      return Promise.resolve({});
    };
  }
  const request = new HttpRequest({ method: "POST", hostname: "example.com" });
  const serFn: (
    op: RestJsonServiceOperations
  ) => __OperationSerializer<RestJsonService, RestJsonServiceOperations, __SmithyException> = (op) => {
    return new TestSerializer();
  };
  const handler = new RestJsonServiceHandler(service, testMux, serFn);
  let r = await handler.handle(request);

  expect(r.statusCode).toBe(400);

  expect(r.headers["content-type"]).toBeDefined();
  expect(r.headers["content-type"]).toBe("application/json");
  expect(r.headers["x-amzn-errortype"]).toBeDefined();
  expect(r.headers["x-amzn-errortype"]).toBe("InvalidGreeting");

  expect(r.body).toBeDefined();
  const utf8Encoder = __utf8Encoder;
  const bodyString = `{
                \"Message\": \"Hi\"
            }`;
  const unequalParts: any = compareEquivalentJsonBodies(bodyString, r.body.toString());
  expect(unequalParts).toBeUndefined();
});

Additionally, I tweaked the serializeError for operations with no modeled errors to immediately throw. Before it was creating an empty switch statement which would cause TypeScript to fail to compile since the errors union is never.

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