Use more expressive error messages when failing out in visudo

[sohomdatta1]

Old error message:

visudo: Permission denied (os error 13)

Newer error message:

visudo: Failed to open existing sudoers file at "/etc/sudoers": Permission denied (os error 13)

(etc)

[sohomdatta1]

Retroactively filling this out since Github didn't freakin prompt me :(

Pull Request Checklist

• I have read and accepted the code of conduct for this project.
• I have tested, formatted and ran clippy over my changes. (I see a warning, but it's not related to the code change)
• I have commented and documented my changes.
• This pull request will fix issue Improved error messages in case of configuration errors #835 where a proper discussion about a solution has taken place.

(#835 was the closest I could find, but I also assumed that this would be considered a small change under the Contributing guidelines)

[squell]

#835 is the relevant issue---improving the error message you quoted was the prime objective.

To fully close #835 we might want to look at error messages in case PAM is misconfigured (e.g. no PAM profile can be loaded due to a missing /etc/pam.d/sudo and no 'default')

note: We currently don't use PR templates; it's appreciated that you took the time to use the checklist, but it's not a hard requirement on this repository. I have some reservations about forcing people to "tick boxes". For example, the 'code of conduct' is not a "contract" you have to "accept", it's simply something we expect. Some of the other things are checked in CI or code review.

[squell]

Thanks for your contribution!