Add message property to IPA's errors and warnings under Python 3

Python 3 removes the "message" attribute from exceptions, in favor
of just calling str().
Add it back for IPA's own exception types.

Reviewed-By: Tomas Babej <tbabej@redhat.com>

ipalib/errors.py

@@ -102,6 +102,8 @@ current block assignments:
             - **5100 - 5999**  *Reserved for future use*
 """
 
+import six
+
 from ipalib.text import ngettext as ungettext
 from ipalib import messages
 from ipaplatform.paths import paths
@@ -124,6 +126,11 @@ class PrivateError(Exception):
             setattr(self, key, value)
         Exception.__init__(self, self.msg)
 
+    if six.PY3:
+        @property
+        def message(self):
+            return str(self)
+
 
 class SubprocessError(PrivateError):
     """
@@ -251,6 +258,11 @@ class PublicError(Exception):
     rval = 1
     format = None
 
+    if six.PY3:
+        @property
+        def message(self):
+            return str(self)
+
 
 class VersionError(PublicError):
     """