Improve role, fail message, and explanation

[dschrempf]

• Use a variable fail_msg (with a message a bit more verbose than the original one).
• Check for fail_msg when getting the explanation. Do not ask for an explanation if the answer was unknown.
• Improve the role messages slightly (input given -> given input; Linux command-> Linux shell command; Don't be too verbose -> Be precise and succinct).

Please let me know, if you only want to merge part of the changes and prefer separate PRs, thank you!

[thomas-endres-tng]

Thanks a lot for this PR. Looks good to me.