better error for invalid output paths

folder_paths.py

@@ -228,8 +228,12 @@ def get_save_image_path(filename_prefix, output_dir, image_width=0, image_height
     full_output_folder = os.path.join(output_dir, subfolder)
 
     if os.path.commonpath((output_dir, os.path.abspath(full_output_folder))) != output_dir:
-        print("Saving image outside the output folder is not allowed.")
-        return {}
+        err = "**** ERROR: Saving image outside the output folder is not allowed." + \
+              "\n full_output_folder: " + os.path.abspath(full_output_folder) + \
+              "\n         output_dir: " + output_dir + \
+              "\n         commonpath: " + os.path.commonpath((output_dir, os.path.abspath(full_output_folder))) 
+        print(err)
+        raise Exception(err)
 
     try:
         counter = max(filter(lambda a: a[1][:-1] == filename and a[1][-1] == "_", map(map_filename, os.listdir(full_output_folder))))[0] + 1