Added error redirection to avoid stack traces.

![screenshot from 2013-09-15 06 49 36](https://f.cloud.github.com/assets/1577944/1144523/0afb8f18-1da5-11e3-8bed-18414a4685b4.png)

Notice that the stack trace on `CTRL+C` are hidden now, but we can still see the warning generated by `powerline-shell`. So we are good :+1: