Change the exception thrown by StorageClass to return unknown

Author: zhuxiaolong37

src/main/java/com/aliyun/oss/model/StorageClass.java

@@ -67,6 +67,6 @@ public static StorageClass parse(String storageClassString) {
             }
         }
 
-        throw new IllegalArgumentException("Unable to parse " + storageClassString);
+        return Unknown;
     }
 }