Override __unicode__ in SaveObjectException

In case of an exception e.g. in save method of the model, the exception is propagated correctly to the form but the user is presented with a generic error message "Please, correct the errors below". Caused by not overriding the __unicode__ method in the SaveObjectException.

It might cause showing unwanted error messages in flexibee_backend (see save_model method in flexibee_backend/is_core/init) which may be easily fixed there.