Add Error message to top of code pane if carto error

Line-based error messages are super fickle. They seem to appear on the wrong line often, and are sometimes offscreen. What seems like a save may not have been. How about just doing this:

Fixes #387

![screen shot 2014-08-19 at 6 24 31 pm](https://cloud.githubusercontent.com/assets/108094/3974072/e4156ae4-27ef-11e4-9847-5d673037484d.png)

cc: @yhahn