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

Minor edits, and added example of deeply nested closure error handling.
Promises absolutely allow for multiple catch() calls!

There isn't anything that forces the programmer to add a catch, but there are a few reasons this is less important than remembering the 'if' statement. Omitting the catch can never result in a null pointer exception, for example.

I should add some text to that effect.