Make errors in container image builds error and show why

[mdales]

I went slightly made trying to work out what was going wrong when a pip dependancy updated and make my linter checks in my build spec fail, but shark would just swallow the error and try to build the rest of the pipeline, eventually failing when it tried to use the image it hand't successfully built.

This change:

• Makes progress stop if an image can't be built
• Puts in the markdown the output of the failed build