Add offline message (bug 1044225)

* Add offline checking. 
* I re-jigged/simplified the error page display a little (I winged this, but if @pwalm has thoughts I'm happy to iterate - and if there's already something defined let me know.)

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)