Update 2014-09-26-The-Exceptional-Doorway.md

Minor edits, and added example of deeply nested closure error handling.
Curious: does the promise pattern allow for multiple `catch()` calls? And is there anything that forces the programmer to add the `catch()`? The same guy who would forget the `if (error)` block may also be likely to forget the `catch()` in a promise chain. Presumably one could write a promise that enforces that practice, whereas enforcing the `if (error)` would be nearly impossible.