Fix fetching of AssertionError message for Python 3

Exceptions in Python 3 no longer contain a `msg` attribute. Instead the
exception message is accessible from the exception object directly.

In order to maintain Python 2 support, attempt to fetch the message from `e.msg`
and use `str(e)` as a default if the attribute does not exist.

Fixes ColtonProvias#28

sqlalchemy_jsonapi/serializer.py

@@ -906,7 +906,7 @@ def patch_resource(self, session, json_data, api_type, obj_id):
             raise ValidationError(str(e.orig))
         except AssertionError as e:
             session.rollback()
-            raise ValidationError(e.msg)
+            raise ValidationError(getattr(e, 'msg', str(e))
         except TypeError as e:
             session.rollback()
             raise ValidationError('Incompatible data type')
@@ -1043,7 +1043,7 @@ def post_collection(self, session, data, api_type):
             raise ValidationError(str(e.orig))
         except AssertionError as e:
             session.rollback()
-            raise ValidationError(e.msg)
+            raise ValidationError(getattr(e, 'msg', str(e)))
         except TypeError as e:
             session.rollback()
             raise ValidationError('Incompatible data type')