Encourage warning messages for `onFulfilled` or `onRejected` being promises

This is probably the most common error I see from beginners using promises on stack overflow (e.g. http://stackoverflow.com/questions/17517593/more-complex-deferred-promise-usage-and-chains-just-arent-clicking-would-love/17525172).  They call the function, rather than passing the function to `.then`.  Since I can't see why anyone would intentionally pass a promise to a `.then` handler and these sorts of functions almost always return promises, I think this could be a big win for helping people get started faster.

P.S. the PR is just there as a discussion point, I'm happy to re-word/move it as appropriate.