Fix failure screen

These changes fix the display of the failure screens so that they look like this:

![screen shot 2014-07-28 at 1 27 49 am](https://cloud.githubusercontent.com/assets/8218578/3716828/4c5c5538-1619-11e4-8156-eb423a8a79c1.png)

Currently in master the text description of the error is out of place onscreen. And the text is also not rendered properly because `miniLock.crypto.worker.onmessage` doesn’t send an error message to the UI callback.

To fix it up I did the following:

1. Updates the position of the notice following recent changes to the <header> position.
2. Revised `miniLock.crypto.worker.onmessage` to pass `operation` AND `errorMessage` to `miniLock.UI.fileOperationHasFailed` so that error details can be displayed onscreen.
3. Revised the crypto worker to post `error` messages instead of a `error` booleans.