Fix error handling for duplicate question ID

http://manage.dimagi.com/default.asp?127310

@mwhite 
I believe [the problem is here](https://github.com/dimagi/Vellum/blob/0c100d3da218c23c997a18d4d6a0a0148d772f65/src/mugs.js#L94). The mug property value is set, change handler fired, and then reset to the previous value if the change handler returned false. Always wondered why it was done that way.