Don't try to handle quiet errors

Hello @nicks, @x-ma, 

Please review the following commits I made in branch 'nathan-deferredErr'.

0a398a756ffc1190ec67ded43afeedb9077dcca9 (2014-10-08 09:36:01 -0700)
Don't try to handle quiet errors

I was slightly wrong in my analysis, it looks like quiet errors are going to be
propogated through just about everything, including the `__deferreds` nodes (which
don't actually have any dependencies).

This is a slight semantic change since it means that error handlers will only
handle errors that happen from their nodes, not those that are propogated.

Another quick solution here is to just not re-throw the error from `handleThunkError`

R=@nicks
R=@x-ma
what makes you think we never actually return the promise to the user anywhere? returning the promise is the whole point of the thunk.