Fix error tooltip(#83)

This fixes the problem we were having showing some errors.

In case of an exception, if we receive an array, we'll join the array with a <br> and send that forward to the template.

In the template we would draw a different "tooltip" for each "stack" message. This never worked correctly as they would overlap eachother. I've changed this to only have 1 tooltip per error, and show the stack inside.

In case of a failure we jsonify the result so this works nicely. Further down the line we might want to improve the look of this JSON object by also prettifying it. But thats another issue.

I've also improved and cleaned up the CSS a bit for the tooltip, and put up a monospaced font to make it look more like a console output.

Demo: http://mozmill-sandbox.blargon7.com/#/remote/report/40742e0746fd767e6d2fd58659a6dd52

@whimboo please review