fix(router): show error screen when view rendering fails

This adds a generic "Unexpected error" page to catch errors during page rendering.

@ryanfeeley I couldn't find any error designs besides https://www.dropbox.com/s/6aoi7vk4jqn4v60/400%20and%20500%20Errors.pdf, and none of those quite fit this error case. As such, the error screen currently looks like this:

![screen shot 2014-07-18 at 2 24 47 pm](https://cloud.githubusercontent.com/assets/3903/3632216/fe3000ac-0ec1-11e4-94e0-6505f579a7e4.png)

Maybe we can spice it up with a cute maintenance robot or something.

Fixes #1365.

@shane-tomlinson r?
I believe that we should wrap this entire page in `<div id="main-content">...</div>`, per #1273 (see [/app/scripts/templates/confirm.mustache](https://github.com/mozilla/fxa-content-server/blob/master/app/scripts/templates/confirm.mustache))

/cc @johngruen for confirmation.