transition.abort() causes 500 responses

As @rwjblue pointed out in #195, aborting a transition in FastBoot causes a `TransitionAborted` error to be thrown by the server, resulting in the request failing and the response returning a `500` error.

Here is an example stack trace:

```
Error: TransitionAborted at handleReject (/code/ember-bugs/dist/fastboot/vendor.js:14132:17)
at tryCatch (/code/ember-bugs/dist/fastboot/vendor.js:63911:14)
at invokeCallback (/code/ember-bugs/dist/fastboot/vendor.js:63926:15)
at publish (/code/ember-bugs/dist/fastboot/vendor.js:63894:9)
at publishRejection (/code/ember-bugs/dist/fastboot/vendor.js:63829:5)
at /code/ember-bugs/dist/fastboot/vendor.js:42159:7 at Queue.invokeWithOnError (/code/ember-bugs/dist/fastboot/vendor.js:10447:18)
at Object.Queue.flush (/code/ember-bugs/dist/fastboot/vendor.js:10502:11)
at Object.DeferredActionQueues.flush (/code/ember-bugs/dist/fastboot/vendor.js:10310:17)
at Object.Backburner.end (/code/ember-bugs/dist/fastboot/vendor.js:10665:25)
at [object Object]._onTimeout (/code/ember-bugs/dist/fastboot/vendor.js:11231:18)
at Timer.listOnTimeout (timers.js:92:15)
```

I'll try to look at this when I get some time. If anyone has any thoughts on what might be causing this, please chime in.