[#565] Descend Bad_AVU_Value from ValueError

... rather than straight from class Exception. This conforms to test_add_obj_meta_empty()'s expectation of
catching a ValueError.

Author: d-w-moore

irods/message/__init__.py

@@ -20,7 +20,7 @@
                                     IntegerProperty, LongProperty, ArrayProperty,
                                     SubmessageProperty)
 
-class Bad_AVU_Field(Exception):
+class Bad_AVU_Field(ValueError):
     pass
 
 _TUPLE_LIKE_TYPES = (tuple, list)