fix(appStart): show an error screen when errors occur during app start

Fixes #1652.

@shane-tomlinson r?
Is this for unit or functional testing? If for functional, could you use something like the "automatedBrowser" trick you use [here](https://github.com/mozilla/fxa-content-server/blob/d2f5212642f4b8275f9eaca94bc0e3503fad5df6/app/scripts/views/base.js#L86).