Wrap try-catch around xhr.status access to fix IE9 JS error c00c023f

In IE9, if the following steps occur:
1. Create a XMLHttpRequest object
2. Assign an onreadystatechanged event handler
3. Execute a request
4. Abort the request before the response has been handled

Then:
Any attempt to read any properties off the XMLHttpRequest will cause the error:
"Could not complete the operation due to error c00c023f"

Solution:
Catch the error and assume the status is 0.

See:
http://stackoverflow.com/questions/7287706/ie-9-javascript-error-c00c023f
http://www.enkeladress.com/article/internetexplorer9jscripterror
Merge pull request #512 from quizlet/fix-ie9-abort-bug

Wrap try-catch around xhr.status access to fix IE9 JS error c00c023f