Add error substate example with dynamic segments.

    The mental model of a route that has a dynamic segment in it is "two layers" as evidenced by https://github.com/emberjs/ember.js/issues/4560. In order to curb the confusion I've explicitly called that out and provided an example of how to accomplish the typical goal.
Merge pull request #1391 from nathanhammond/patch-1

Add error substate example with dynamic segments.