grub-probe: When a device has been specified WITHOUT option '-d', grub-probe should display an additional user-friendly advice message

[Etienne URBAH]

When a device has been specified to 'grub-probe' WITHOUT option '-d', 'grub-probe' systematically displays following error message :
grub-probe: error: failed to get canonical path of `udev'.

This error message is logical when considering the internal point of view of 'grub-probe', but it is NOT user-friendly.

Therefore, I suggest that 'grub-probe' displays an additional user-friendly advice message like the following one :
grub-probe: advice: specifying a device requires option '-d'.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: grub-common 2.02~beta2-36ubuntu3
ProcVersionSignature: Ubuntu 4.4.0-22.39-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: X-Cinnamon
Date: Thu May 19 17:23:33 2016
InstallationDate: Installed on 2014-11-03 (562 days ago)
InstallationMedia: Ubuntu-GNOME 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
SourcePackage: grub2
UpgradeStatus: Upgraded to xenial on 2016-05-09 (9 days ago)