misleading feedback when reading of device file fails due to unsufficient permissions

[Karl-Philipp Richter]

When one wants to rip from an iso image associated with a loop device (using `losetup`), the reading of the DVD table of content fails with the error

    Job 'Read TOC (lsdvd)' failed with error message:
    Error reading table of contents. Please check your DVD device settings in the Preferences and don't forget to put a DVD in the drive.

Although it's the task of `libdvdread` to provide a useful error message (pointing out that it's a permission issue which is much less vague than the message above), `dvdrip` should compensate that and provide sufficient feedback.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: dvdrip 1:0.98.11-0ubuntu5
Uname: Linux 4.0.1-040001-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat May 23 19:16:20 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (50 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: dvdrip
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)