#679 - Fix Rails double render issues.

[Initial fix for first issue of #679. This is quite a generic fix and might fix the other two issues mentioned in #679, but that is not yet checked]

During config save, there were some cases when a double render would happen. In this case,
config change gets merged successfully, but since the original job name is no longer valid,
it fails to load it.

The actual error message should be: "Pipeline not found", but that's hard to get right, generically (what if the stage is the one that is renamed? What if it is something else?). So, the error now shown looks like this:

![screen shot 2014-10-29 at 4 48 34 pm](https://cloud.githubusercontent.com/assets/172235/4835226/549f05ca-5fb5-11e4-9384-7903b8f1bbf7.png)

I think it's better than what was showing up earlier. Not perfect, but better?