Add error handling for no repo_url

@slicebo123 please review, @rappo FYI

This should handle when there is no repo_url, and display a plain text step instead.

![screen shot 2014-05-06 at 4 17 12 pm](https://cloud.githubusercontent.com/assets/506085/2896885/9a6822d8-d574-11e3-958b-de4726b4f85f.png)

Relates to: https://github.com/bountysource/frontend/issues/594