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
We're actually calling `console.warn` with the error after this `.fail` function gets executed.  Execution gets completed, it just gets completed early (we seem to drop out), and so we do never get to the `.then` above, but we do finish executing.

I don't know that it actually makes sense to propagate the error to the lazy graph evaluation because we never actually return the promise to the user anywhere (so you couldn't really depend on it).