Error formatting

See design background in #54.

Here are some samples for both light and dark terminals:

![](http://cl.ly/image/0l2n3t3K1E3v/Image%202014-01-09%20at%202.27.41%20PM.png)

![](http://cl.ly/image/0G1y1c2Z2j0A/Screen%20Shot%202014-01-09%20at%202.26.30%20PM.png)

There are not yet warning messages anywhere, though, since heroku-go doesn't yet expose them.