some git error cleanup

Improve our error message when cloning a repo from bitbucket (to help improve the messages for #2813).

old dialog:
![screen shot 2014-07-07 at 7 40 21 am](https://cloud.githubusercontent.com/assets/1269969/3496676/efb98d08-05e4-11e4-9196-0db941e352cb.png)

new dialog:
![screen shot 2014-07-07 at 7 39 50 am](https://cloud.githubusercontent.com/assets/1269969/3496668/eae88c2a-05e4-11e4-9d6d-7852ef4d7fca.png)


@gaurave

@srsaroop any sample repository in mind that would be hosted on dart-lang or googlechrome repo?