Change error message with invalid render usage (#12265)

* Change error message with invalid render usage

This change disambiguates this error condition with a different error with the same error message at 789baf4c1f/src/mbgl/map/map.cpp (L157-L160)

* fix test expectation

platform/node/src/node_map.cpp

@@ -398,7 +398,7 @@ void NodeMap::Render(const Nan::FunctionCallbackInfo<v8::Value>& info) {
     }
 
     if (nodeMap->req) {
-        return Nan::ThrowError("Map is currently rendering an image");
+        return Nan::ThrowError("Map is currently processing a RenderRequest");
     }
 
     try {