Fix error handling for duplicate question ID

http://manage.dimagi.com/default.asp?127310

@mwhite 
Not a huge deal but it would be nice to not have the information about how the input name is formed duplicated like this.  Would be nice to do `_this.getCurrentMugInput('nodeID').select().focus()` or something like that.  I know I did this in the tests but I consider that slightly less bad because they are guaranteed to break if someone changes the way inputs are identified, whereas this could easily get missed.

Also, I guess I shouldn't really expect you to maintain this, but there is currently (although I haven't tested whether there are any conflicts involving modals) the ability to have multiple instances of vellum on the same page, so to maintain that you would instead do:
```
_this.$fd.find("input[name=property-nodeID]");
```
I suppose you could probably argue that would be faster too, due to not having to traverse the entire DOM.