using mirror without source packages ends in cryptic error

[manut]

If a mirror is used, that doesn't contain the source packages, the source CDROM can't be generated.
However the resulting error messages are very confusing.

Please improve validation that runs before the buld, that it checks if the source mirrors are availbable
if --skip-build-sources wasn't specified.

---

Installing GRUB boot loader ... 16%... 33%... 50%... 66%... 83%... 100%
The system is going down NOW!.. 13%... 22%... 31%... 40%... 50%... 63%... 72%... 81%... 90%
Sent SIGTERM to all processes
Sent SIGKILL to all processes
Requesting system reboot
[ 610.013885] reboot: Restarting system
mkdir -p .stamps
touch .stamps/stamp-install-initial-image

---

XML file is inavlid: Repository http://10.0.2.2//debian/dists/jessie/Release can not be validated

termi@Ubuntu12:~/elbe-v1$ ./elbe initvm submit ./testlinux.xml
Build started, waiting till it finishes
Tue Mar 8 15:03:31 2016-- project still busy, waiting
Tue Mar 8 15:03:36 2016-- project still busy, waiting
Tue Mar 8 15:03:41 2016-- project still busy, waiting
...
Build finished !

Server returned error:

OSError: [Errno 2] No such file or directory: '/var/cache/elbe/59969b01-165f-400d-8940-3bb0006401e0/validation.txt'
Project failed to generate validation.txt
Getting log.txt
Enqueueing project for build
Build started

ELBE Report for Project elbe-gnome3

report timestamp: 20160308-140329
running cmd +mkdir -p "/var/cache/elbe/59969b01-165f-400d-8940-3bb0006401e0/chroot"+

debootstrap log

getting output from cmd +dpkg --print-architecture+

running cmd +debootstrap --arch=amd64 "jessie" "/var/cache/elbe/59969b01-165f-400d-8940-3bb0006401e0/chroot" "http://10.0.2.2//debian"+

I: Retrieving Release
I: Retrieving Release.gpg
...
I: Configuring systemd...

I: Base system installed successfully.

running cmd +chroot /var/cache/elbe/59969b01-165f-400d-8940-3bb0006401e0/chroot debconf-set-selections < /var/cache/elbe/59969b01-165f-400d-8940-3bb0006401e0/chroot/var/cache/elbe/preseed.txt+
update cache failed
Build failed !!!

Failed to build the Apt Cache.
Not all Mirror Index Files could be downloaded.
Probable cause might be:

• Problems with Internet Connection
• Broken mirrors

[manut]

fixed by adding a validator 555fa1d