add a necessary check for checkpoints

add a necessary check for checkpoints, if you miss the file prefix and it will not execute the checkpoints codes

ppocr/utils/save_load.py

@@ -109,9 +109,14 @@ def init_model(config, program, exe):
     """
     checkpoints = config['Global'].get('checkpoints')
     if checkpoints:
+        if os.path.exists(checkpoints + '.pdparams'):
             path = checkpoints
             fluid.load(program, path, exe)
             logger.info("Finish initing model from {}".format(path))
+        else:
+            raise ValueError(
+                "Model checkpoints  {} does not exists,"
+                "check if you lost the file prefix .".format(checkpoints + '.pdparams'))
 
     pretrain_weights = config['Global'].get('pretrain_weights')
     if pretrain_weights: