unhelpful feedback "Error: Incorrect command line"

[Karl-Philipp Richter]

When specifying an incorrect command line `p7zip`, e.g. `7zr a -p -mhe=on archive.7zr .`, only `Error: Incorrect command line` is displayed which is unhelpful because it doesn't explain why the command line is incorrect.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: p7zip 9.20.1~dfsg.1-4.2
Uname: Linux 4.2.5-040205-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
Date: Tue Feb 16 15:57:10 2016
Dependencies:
 gcc-5-base 5.2.1-22ubuntu2
 libc6 2.21-0ubuntu4
 libgcc1 1:5.2.1-22ubuntu2
 libstdc++6 5.2.1-22ubuntu2
InstallationDate: Installed on 2015-04-20 (301 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: p7zip
UpgradeStatus: Upgraded to wily on 2015-10-26 (113 days ago)