Better error pages in the login flow

Assign: @natebrennand 

The "log out" button will disconnect their account, so that they will have to re-auth.

If we didn't whitelist their email:
![screen shot 2014-09-28 at 1 43 13 am](https://cloud.githubusercontent.com/assets/2433509/4433388/ae2363ae-46d6-11e4-8f8c-801198f82f58.png)

Any other errors:
![screen shot 2014-09-28 at 1 42 59 am](https://cloud.githubusercontent.com/assets/2433509/4433389/ae23b6e2-46d6-11e4-965c-76e19e7238d3.png)

Successful login:
![screen shot 2014-09-28 at 1 51 23 am](https://cloud.githubusercontent.com/assets/2433509/4433390/ae2ace14-46d6-11e4-803a-7cf6ba3f390c.png)

The reason why this is a problem is that after it makes the ajax request to the server to login, the Google JS client will automatically attempt to redirect to the next url, which (if there are errors) will redirect back to this page.  So I tried to show the errors in the `error` part of the `$.ajax` call, but then the page refreshes (the `next` url failed and dumped us back at `/admin/login`).   Because we need to know whether or not there is an error on page refresh, just creating variables or changing HTML in the `$.ajax` callback won't be enough, hense the need for `window.error`