Add offline message (bug 1044225)

* Add offline checking. 
* Re-jig the error display a little

Not sure if we need to test for packaged support of if we're happy to have this run by default like the system date checking?

![firefox_marketplace](https://cloud.githubusercontent.com/assets/1514/3923683/716dedfe-23d5-11e4-9207-c6b0684a8577.png)

![firefox_marketplace](https://cloud.githubusercontent.com/assets/1514/3923682/6d4ba432-23d5-11e4-871b-d1e0b618e224.png)

![firefox_marketplace](https://cloud.githubusercontent.com/assets/1514/3923676/5c3e5cca-23d5-11e4-827f-6770f1f00a0a.png)

![firefox_marketplace](https://cloud.githubusercontent.com/assets/1514/3923681/68ac4300-23d5-11e4-8495-78894ead5d5b.png)