Add default 404 error page

src/HttpContext.h

@@ -171,6 +171,10 @@ private:
                 /* Route the method and URL */
                 selectedRouter->getUserData() = {(HttpResponse<SSL> *) s, httpRequest};
                 if (!selectedRouter->route(httpRequest->getCaseSensitiveMethod(), httpRequest->getUrl())) {
+                    /* We don't care if it reaches the client or not */
+                    us_socket_write(SSL, (us_socket_t *) s, httpErrorResponses[HTTP_ERROR_404_FILE_NOT_FOUND].data(), (int) httpErrorResponses[HTTP_ERROR_404_FILE_NOT_FOUND].length(), false);
+                    us_socket_shutdown(SSL, (us_socket_t *) s);
+
                     /* We have to force close this socket as we have no handler for it */
                     us_socket_close(SSL, (us_socket_t *) s, 0, nullptr);
                     return nullptr;

src/HttpErrors.h

@@ -25,7 +25,8 @@ namespace uWS {
 enum HttpError {
     HTTP_ERROR_505_HTTP_VERSION_NOT_SUPPORTED = 1,
     HTTP_ERROR_431_REQUEST_HEADER_FIELDS_TOO_LARGE = 2,
-    HTTP_ERROR_400_BAD_REQUEST = 3
+    HTTP_ERROR_400_BAD_REQUEST = 3,
+    HTTP_ERROR_404_FILE_NOT_FOUND = 4
 };
 
 /* Returned parser errors match this LUT. */
@@ -33,7 +34,8 @@ static const std::string_view httpErrorResponses[] = {
     "", /* Zeroth place is no error so don't use it */
     "HTTP/1.1 505 HTTP Version Not Supported\r\n\r\n<h1>HTTP Version Not Supported</h1><p>This server does not support HTTP/1.0.</p><hr><i>uWebSockets/20 Server</i>",
     "HTTP/1.1 431 Request Header Fields Too Large\r\n\r\n<h1>Request Header Fields Too Large</h1><hr><i>uWebSockets/20 Server</i>",
-    "HTTP/1.1 400 Bad Request\r\n\r\n<h1>Bad Request</h1><hr><i>uWebSockets/20 Server</i>"
+    "HTTP/1.1 400 Bad Request\r\n\r\n<h1>Bad Request</h1><hr><i>uWebSockets/20 Server</i>",
+    "HTTP/1.1 404 File Not Found\r\n\r\n<h1>File Not Found</h1><hr><i>uWebSockets/20 Server</i>"
 };
 }