RANGER-5338: Make XMLUtils's error message more informative (apache#685)

Author: okumin

agents-common/src/main/java/org/apache/ranger/plugin/util/XMLUtils.java

@@ -51,7 +51,7 @@ public static void loadConfig(String configFileName, Map<Object, Object> propert
         try (InputStream input = getFileInputStream(configFileName)) {
             loadConfig(input, properties);
         } catch (Exception e) {
-            LOG.error("Error loading : ", e);
+            LOG.error("Error loading : {}", configFileName, e);
         }
     }
 
@@ -135,6 +135,9 @@ private static InputStream getFileInputStream(String path) throws FileNotFoundEx
             }
         }
 
+        if (ret == null) {
+            throw new FileNotFoundException(path + " is not found");
+        }
         return ret;
     }
 }