Using build message file for all statuses.

This is a bit of an RFC, not necessarily a candidate for merging (at least immediately).

I wanted to add a custom message file for successful builds. After plumbing that through, I decided that the most logical way was to just have a single build message fail that could be used if it was available in case of both successful and unsuccessful builds. This made everything much simpler, and I think it fits the use cases a bit better also (at least my intended use case).

I've never done anything with Jenkins plugins before, so all feedback welcome and appreciated :).