Add HttpService::Response::sendError() convenience method.

c++/src/kj/compat/http.c++

@@ -2643,6 +2643,18 @@ kj::Own<HttpService> newHttpService(HttpClient& client) {
 
 // =======================================================================================
 
+kj::Promise<void> HttpService::Response::sendError(
+    uint statusCode, kj::StringPtr statusText, const HttpHeaders& headers) {
+  auto stream = send(statusCode, statusText, headers, statusText.size());
+  auto promise = stream->write(statusText.begin(), statusText.size());
+  return promise.attach(kj::mv(stream));
+}
+
+kj::Promise<void> HttpService::Response::sendError(
+    uint statusCode, kj::StringPtr statusText, const HttpHeaderTable& headerTable) {
+  return sendError(statusCode, statusText, HttpHeaders(headerTable));
+}
+
 kj::Promise<void> HttpService::openWebSocket(
     kj::StringPtr url, const HttpHeaders& headers, WebSocketResponse& response) {
   class EmptyStream final: public kj::AsyncInputStream {

c++/src/kj/compat/http.h

@@ -518,6 +518,17 @@ public:
     //
     // `statusText` and `headers` need only remain valid until send() returns (they can be
     // stack-allocated).
+
+    kj::Promise<void> sendError(uint statusCode, kj::StringPtr statusText,
+                                const HttpHeaders& headers);
+    kj::Promise<void> sendError(uint statusCode, kj::StringPtr statusText,
+                                const HttpHeaderTable& headerTable);
+    // Convenience wrapper around send() which sends a basic error. A generic error page specifying
+    // the error code is sent as the body.
+    //
+    // You must provide headers or a header table because downstream service wrappers may be
+    // expecting response headers built with a particular table so that they can insert additional
+    // headers.
   };
 
   virtual kj::Promise<void> request(