overwrite output directories if needed

examples/run_bert_squad.py

@@ -179,7 +179,7 @@ def main():
                 "If `do_predict` is True, then `predict_file` must be specified.")
 
     if os.path.exists(args.output_dir) and os.listdir(args.output_dir) and args.do_train and not args.overwrite_output_dir:
-        raise ValueError("Output directory () already exists and is not empty.")
+        raise ValueError("Output directory {} already exists and is not empty. Use --overwrite_output_dir to overcome.".format(args.output_dir))
     if not os.path.exists(args.output_dir):
         os.makedirs(args.output_dir)