Add postbuild message handling infrastructure

This adds the capability to handle post-build messages and provides an initial user of that in git-stash handling.

What is looks like (on failure, obviously):

What it looks like in a more normal build:

We'd probably want to find other error types that would warrant this kind of post-build alert. But I think the git-stash thing is a good first candidate because it pops up more frequently for our users so it would be good to fix (and to warn about when we can't fix).