Merge #6316

6316: Display reason to require sudo r=colby-swandale a=okkez

This is useful for non-interactive installation with bundler.

### What was the end-user problem that led to this PR?

treasure-data/omnibus-td-agent#166

I could not notice that bundler needs sudo privilege from logs.
So I checked bundler code.

### What was your diagnosis of the problem?

Bundler does not show the reason to need sudo privilege.

### What is your fix for the problem, implemented in this PR?

Display reason to require sudo.

### Why did you choose this fix out of the possible options?

If bundler displays reason to require sudo, we can notice permission problems as soon as possible.


Co-authored-by: Kenji Okimoto <okimoto@clear-code.com>