submit JS errors to Ashes on `window.onerror` (bug 965040)

Raven.js is a bit too much for my liking. I tried submitting the errors to [Ashes](https://github.com/mozilla/ashes/), but unfortunately Ashes won't accept any arbitrary data format.

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![screenshot 2014-10-16 16 23 41](https://cloud.githubusercontent.com/assets/203725/4672254/ef9ba2ec-558b-11e4-95ea-cacff0ad8a3b.png)
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![screenshot 2014-10-16 16 23 51](https://cloud.githubusercontent.com/assets/203725/4672255/ef9e0820-558b-11e4-8492-3d3fbb6f6777.png)
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make these changes to Ashes: 

1. add an "Errors" tab [in `script.js` here](https://github.com/mozilla/ashes/blob/285d6ca/static/script.js#L83-L90), following the "Settings" mapping (`<dt>` for each key and `<dd>` for each value in the `errors` object)
2. add an endpoint (behind auth, obviously) to the [Ashes API](https://ashes.paas.allizom.org/) that lists all reports with their IDs (currently, you have to know the URL to be able to see its page - and you must be logged in)