Setting exit status to worst severity

[simoncozens]

We currently have a situation where certain FAILs are actual showstoppers and should cause the CI to fail and a PR to be unmergable, but certain FAILs are "judgment calls" and should be manually reviewed before (potentially) merging. Fontbakery has "severity" metadata for (some) checks, but if you're a CI, there's no easy access to that information.

One simple and backward-compatible way to access it would be to collect a worst_check_severity in the same way we collect a worst_check_status, and use that worst severity as the process exit status. So any bad checks would continue to cause a non-zero exit status, but you could use the exit status to retrieve the severity of the most severe check and then use it to make a decision about whether to fail the CI.