Improve support for investigation of build failures, via status of upstream dependencies

[mascguy]

When investigating build failures caused by failing dependencies, this requires traveling down the rabbit hole, inspecting why an upstream port is failing.

This is routinely due to dependencies, which are only supported on newer macOS releases. But it can be a tedious and time-consuming effort to determine whats-what, particularly when one has to delve down three or more levels.

Ideally we should provide a straightforward view - based on dependency build status - to simplify this.

Implementing issue #344 would also assist here.

[Dave-Allured]

When a port goes to "failed install-dependency", we need a simple mechanism to remember the name of the upstream broken dependency. The builder log with this information gets scrubbed after a few weeks, so that blamed port name becomes hard to recover.

Storage for only one blamed dependency name per port, and per builder, is needed (M by N). By any chance, does a slot already exist for this blamed port name, on either builders or webapp infrastructure?

I suggest this blamed port name storage should be added in one of those locations. This would be more economical and user friendly than retaining builder logs.

I also suggest that webapp be updated to show this blamed port name up front, right next to "failed install-dependency" in the build status column on a port's builds tab.