Tag: Move error handling code to widget

The error message was handled in cmds.py while the success code was
handled in createtag.py. This will make it harder to maintain later.

This change move the error handling code to the same location as success
handling code.

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: Minarto Margoliono <lie.r.min.g@gmail.com>