DeserializationError: more meaningful message

acme/acme/jose/errors.py

@@ -8,6 +8,10 @@ class Error(Exception):
 class DeserializationError(Error):
     """JSON deserialization error."""
 
+    def __str__(self):
+        return "Deserialization error: {0}".format(
+            super(DeserializationError, self).__str__())
+
 
 class SerializationError(Error):
     """JSON serialization error."""