Improve error message "generic failure"

[Karl-Philipp Richter]

Invoking `saslpasswd2 -f /var/run/saslauthd/mux -c username` can cause the error message `saslpasswd2: generic failure` after entering the password and repeating it. That's as unhelpful as it can be. Explain what went wrong and why.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: sasl2-bin 2.1.26.dfsg1-11
Uname: Linux 4.0.0-040000-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.14.7-0ubuntu8.4
Architecture: amd64
Date: Wed Apr 22 20:08:19 2015
InstallationDate: Installed on 2015-04-20 (2 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: cyrus-sasl2
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.default.saslauthd: 2015-04-22T19:57:04.387349