Add explanatory tooltip or restyle Live Development's pink line number error marker

[peterflynn]

We've gotten a number of reports from people being confused about this indicator. E.g. http://forums.adobe.com/message/6203592#6203592

If we added a tooltip it might make it a little easier to figure out what it means. Unfortunately, while markText() lets you assign a tooltip, addLineClass() doesn't... so this would require a CM patch (albeit one that's probably generally useful).

Or we could change the style to look more like a traditional gutter error badge. I'm not sure why we decided against that originally... but using a gutter marker would make it instantly recognizable.

[redmunds]

This is closely related to the issues raised and solutions discussed in #7126.

[peterflynn]

Nominating for 1.0 as it seems like a high-profile usability issue (along with #7126).

[njx]

Reviewed, leaving in 1.0.