fix(httpBackend): should not read statusText when request is aborted

Request Type: bug

How to reproduce: Abort an XHR request in IE9. Observe "Error c00c023f" in the console.

Component(s): $http

Impact: medium

Complexity: small

**Detailed Description:**

Commit 1d2414ca93a0340840ea1e80c48edb51ec55cd48 introduced a regression which breaks IE9 by accessing the `statusText` property of an aborted xhr request. 
There's been a similar problem earlier which was fixed with commit 6f1050df4fa885bd59ce85adbef7350ea93911a3.

This PR changes the behavior to only access the `statusText` property if the request wasn't aborted.
I'll make that change as I merge this in.