Add visual indicator when a fatal failure occurs

When a fatal failure occurs, a error indicator is added to the status bar, flashes red once, and then turns back to transparent background.

A modal dialog will open when the indicator is clicked, reporting the error, and providing a way to go to the error in the document.

The status bar indicator will go away as soon as Reporter.prototype.checkFatal returns false (no more fatal error).

The formatting and text can probably be worked on, but this is my initial commit for this subject.
Could you create a gist with those two functions (linterReporter.checkFatal and the errorIndicator listener) for me to look at?

Or do you want me to push the Mustache changes (which I have finished) and then we can work from there?