Stop using deprecated 'message' attribute in Exception

The 'message' attribute has been deprecated and removed
from Python3.
For more details, please check:
https://www.python.org/dev/peps/pep-0352/

Change-Id: Ia304170faa6d3a6cec63b8f874101db7559176e5

designatedashboard/dashboards/project/dns_domains/forms.py

@@ -13,6 +13,7 @@
 # under the License.
 import functools
 import re
+import six
 
 from designateclient import exceptions as designate_exceptions
 from django.core.exceptions import ValidationError  # noqa
@@ -52,8 +53,8 @@ def handle_exc(func):
             msg = ""
             data = {}
 
-            if ex.message is not None:
-                data['message'] = ex.message
+            if six.text_type(ex) is not None:
+                data['message'] = six.text_type(ex)
                 msg += "Error: %(message)s"
             else:
                 data["type"] = ex.type