Point user at `test_command` failure outputs

[ckreibich]

When a package's test cases fail, zkg produces zkg.test_command.stdout and zkg.test_command.stderr files in the package's clone in zkg's scratch space — but the user never learns about that, not even when cranking up the debug level. We should report the location of those files, and possibly show the problems immediately if they're not voluminous.