Fix error handling for duplicate question ID

http://manage.dimagi.com/default.asp?127310

@mwhite 
There seem to be two functional changes in this PR.

1. fix the broken `name` reference which was a refactoring fail and would have definitely caused a bug.
2. effectively change `length > 0` to `length > 1` in the above statement, which is necessary due to an oversight on my part when adding the ability to return false from a change handler to abort the change, where the value is actually changed first and then reset on a false return value.

As it is, getMugChildrenByNodeID() confusingly returns the mug we are currently testing to decide whether it can get that nodeID, due to this fact.  This deserves at least a comment, but there must be some solution that wouldn't require this essentially incorrect line.  Perhaps separate functions for testing the validity of the change and handling it.