Add errorschanged event trigger in showErrors / clearErrors

[zyzof]

The existing events are triggered before the error content is added to the DOM if the "add error" operation is deferred.

This change provides a way to detect once the error information has been added.

In my case, this allowed me to display error information outside of the form-group, by hiding the .help-block.with-errors within the form-group and keeping its contents synced with an external element.