[server] fix unicode conversion capability in UndoException
By overriding the __unicode__ method, we ensure unicode builtin will return
Unicode object even for python versions that don't have a __unicode__ method by
default (python < 2.6)
In previous versions, when this method is not defined, string conversion is
attempted, and the result of string conversion is converted to Unicode using
the system default encoding which is ascii the most of the time.
sync_schema_props_perms('CWSource', syncprops=False)