Added an SDL error to SDL_GameControllerMapping* functions

src/joystick/SDL_gamecontroller.c

@@ -1614,6 +1614,8 @@ SDL_GameControllerMappingForIndex(int mapping_index)
             return CreateMappingString(mapping, mapping->guid);
         }
         --mapping_index;
+    } else {
+        SDL_SetError("Mapping not available");
     }
     return NULL;
 }
@@ -1627,6 +1629,8 @@ SDL_GameControllerMappingForGUID(SDL_JoystickGUID guid)
     ControllerMapping_t *mapping = SDL_PrivateGetControllerMappingForGUID(guid, SDL_FALSE);
     if (mapping) {
         return CreateMappingString(mapping, guid);
+    } else {
+        SDL_SetError("Mapping not available");
     }
     return NULL;
 }