Fix error message of load_inference_model (#24314)

* fix error message, test=develop

* add unittest, test=develop

python/paddle/fluid/io.py

@@ -1371,7 +1371,7 @@ def load_inference_model(dirname,
     if dirname is not None:
         load_dirname = os.path.normpath(dirname)
         if not os.path.isdir(load_dirname):
-            raise ValueError("There is no directory named '%s'", dirname)
+            raise ValueError("There is no directory named '%s'" % dirname)
 
         if model_filename is None:
             model_filename = '__model__'

python/paddle/fluid/tests/unittests/test_inference_model_io.py

@@ -192,5 +192,13 @@ class TestInstance(unittest.TestCase):
                           [MODEL_DIR, ["x", "y"], [avg_cost], [], cp_prog])
 
 
+class TestLoadInferenceModelError(unittest.TestCase):
+    def test_load_model_not_exist(self):
+        place = core.CPUPlace()
+        exe = executor.Executor(place)
+        self.assertRaises(ValueError, load_inference_model,
+                          './test_not_exist_dir', exe)
+
+
 if __name__ == '__main__':
     unittest.main()