Provide a way to explain to users what's causing a build to fail

[nickcharlton]

I'd wanted to roll this out in Administrate for a long time, as we started bundling our assets in the Gem itself. Whilst previously, I felt like it was pretty clear what was different in all the other places I'd introduced this, when working on thoughtbot/administrate#2680 this was less the case.

It'd be good to be able to provide some explanation, and better commands to run to fix the problem that is causing the build to fail. This could go in the existing step summary output.