core: display an error message in case of invalid size with command /window resize

src/core/core-command.c

@@ -8170,6 +8170,8 @@ COMMAND_CALLBACK(window)
                 gui_window_resize (ptr_tree, number);
             }
         }
+        else
+            COMMAND_ERROR;
         return WEECHAT_RC_OK;
     }