[java] Improving exception when server returns 504 (#9354)

Add a special case in W3CHttpResponseCodec handling 504 Gateway Time-out errors.

Before, the end user used to see a pretty unfriendly error:

> JsonException: Unable to determine type from: <. Last 1 characters read: <

Author: alb-i986

java/client/src/org/openqa/selenium/remote/codec/w3c/W3CHttpResponseCodec.java

@@ -20,8 +20,8 @@
 import static com.google.common.base.Strings.nullToEmpty;
 import static com.google.common.net.HttpHeaders.CONTENT_TYPE;
 import static java.net.HttpURLConnection.HTTP_BAD_METHOD;
+import static java.net.HttpURLConnection.HTTP_GATEWAY_TIMEOUT;
 import static java.net.HttpURLConnection.HTTP_INTERNAL_ERROR;
-import static java.net.HttpURLConnection.HTTP_OK;
 import static org.openqa.selenium.json.Json.MAP_TYPE;
 import static org.openqa.selenium.json.Json.OBJECT_TYPE;
 import static org.openqa.selenium.remote.http.Contents.string;
@@ -90,6 +90,9 @@ public Response decode(HttpResponse encodedResponse) {
       if (HTTP_BAD_METHOD == encodedResponse.getStatus()) {
         response.setStatus(ErrorCodes.UNKNOWN_COMMAND);
         response.setValue(content);
+      } else if (HTTP_GATEWAY_TIMEOUT == encodedResponse.getStatus()) {
+        response.setStatus(ErrorCodes.UNHANDLED_ERROR);
+        response.setValue(content);
       } else {
         Map<String, Object> obj = json.toType(content, MAP_TYPE);