improve logging when Xvnc command isn't present

[Karl-Philipp Richter]

Currently the absense of `Xvnc` causes errors in the form of

    [20150317-17:00:34] [INFO ] starting Xvnc session...
    [20150317-17:00:34] [CORE ] error starting X server - user putty - pid 4992
    [20150317-17:00:34] [DEBUG] errno: 2, description: No such file or directory
    [20150317-17:00:34] [DEBUG] execve parameter list: 16
    [20150317-17:00:34] [DEBUG] argv[0] = Xvnc
    [20150317-17:00:34] [DEBUG] argv[1] = :10
    [20150317-17:00:35] [DEBUG] argv[2] = -geometry
    [20150317-17:00:35] [DEBUG] argv[3] = 1024x768
    [20150317-17:00:35] [DEBUG] argv[4] = -depth
    [20150317-17:00:35] [DEBUG] argv[5] = 16
    [20150317-17:00:35] [DEBUG] argv[6] = -rfbauth
    [20150317-17:00:35] [DEBUG] argv[7] = /home/putty/.vnc/sesman_putty_passwd
    [20150317-17:00:35] [DEBUG] argv[8] = -bs
    [20150317-17:00:36] [DEBUG] argv[9] = -ac
    [20150317-17:00:36] [DEBUG] argv[10] = -nolisten
    [20150317-17:00:36] [DEBUG] argv[11] = tcp
    [20150317-17:00:36] [DEBUG] argv[12] = -localhost
    [20150317-17:00:36] [DEBUG] argv[13] = -dpi
    [20150317-17:00:36] [DEBUG] argv[14] = 96
    [20150317-17:00:36] [DEBUG] argv[15] = (null)
    [20150317-17:00:44] [ERROR] X server for display 10 startup timeout
    [20150317-17:00:44] [ERROR] X server for display 10 startup timeout
    [20150317-17:00:44] [INFO ] starting xrdp-sessvc - xpid=4992 - wmpid=4991
    [20150317-17:00:44] [ERROR] another Xserver is already active on display 10
    [20150317-17:00:44] [DEBUG] aborting connection...

to be logged to `/var/log/xrdp-sesman.log` where at least "another Xserver is already active on display 10" is confusing because it isn't the reason for the command failing (it's the absense of `Xvnc`).

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: xrdp 0.6.1-1
Uname: Linux 3.19.1-031901-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Mar 17 17:03:24 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-01-26 (49 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: xrdp
UpgradeStatus: No upgrade log present (probably fresh install)

[Lenin (gagarin)]

take this upstream please