No good way for errors to show how to run the same command again

[Kyle Fazzari]

We're starting to introduce errors that need to say something like "please try again." There is a desire (see https://github.com/snapcore/snapcraft/pull/1906#discussion_r165534096) to print the command that was run instead of saying "try again." However, we have no generic way to do this. Click doesn't appear to have a way to give you essentially argv[1:] back, and actually using argv is fragile since it's unpredictable (could be run by tests, etc.)