Bake: Add summary of why build failure happened

[ciaranmcnulty]

Currently when baking lots of targets it's common for the logs to contain:

• Lots of successful build lines
• Some error
• Lots of other lines from the parallel jobs being cancelled

When looking at something like CI output (probably with --progress=plain) it can be hard to locate the source of the error

It would be great to have a summary that explained clearly:

• Which target failed
• Which stage failed
• What line failed
• What the error output was (in the case of a RUN failure)

Importantly, it would be great to have this at the very end of the output just before buildx exits

[ciaranmcnulty]

I'm not sure if there's a case where two failures could happen at the same time? If so we should just summarise both

[Ocramius]

Would be good to have the STDERR/STDOUT of the failed step highlighted, ideally with some sort of delimiter before/after.

Looking into build failures is indeed not really fun, when it becomes an exercise in squinting :D