Detect if build in queue fails and ping PR

[rylev]

If a build fails to build, and there's a PR associated with that build, we should post a comment in that PR, indicating as such.

Based on a conversation had here.

[Mark-Simulacrum]

We can likely start by showing the regression commit on https://perf.rust-lang.org/status.html, in addition to the latest build to have the bug.