missing mplayer command only reported on command line

[Karl-Philipp Richter]

If one starts the playback of a chapter and `mplayer` isn't installed, there's feedback on the command line only. In the worst case the user has no idea what's happening and why the playback doesn't begin.

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 19:11:18 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)