utils/serializer: Fix exception handling in Python3

Allow for the fact that exceptions do not have a 'message' attribute in
Python3.

wa/utils/serializer.py

@@ -231,7 +231,7 @@ class yaml(object):
             lineno = None
             if hasattr(e, 'problem_mark'):
                 lineno = e.problem_mark.line  # pylint: disable=no-member
-            raise SerializerSyntaxError(e.message, lineno)
+            raise SerializerSyntaxError(e.args[0] if e.args else str(e), lineno)
 
     loads = load