Do not purge previous image if ISO build fails

[Jean-Baptiste Lallement]

If the build of the ISO fails previous images are still purged and current build directory on cdimage.u.c is empty.

for example from https://people.canonical.com/~ubuntu-archive/cd-build-logs/ubuntu/cosmic/daily-live-20180621.log

cpio: premature end of archive
Makefile:636: recipe for target '/srv/cdimage.ubuntu.com/scratch/ubuntu/cosmic/daily-live/tmp/cosmic-amd64/packages-stamp' failed
make: *** [/srv/cdimage.ubuntu.com/scratch/ubuntu/cosmic/daily-live/tmp/cosmic-amd64/packages-stamp] Error 1
ERROR WHILE BUILDING OFFICIAL IMAGES !!
===== Publishing =====
Thu Jun 21 07:49:15 UTC 2018
No desktop image for amd64!
No images produced!
===== Purging old images =====
Thu Jun 21 07:49:15 UTC 2018
Purging ubuntu/daily-live images older than 1 day ...
Purging ubuntu/daily-live/20180619

20180619 is purged, it leaves 20180620 and 20180621 on cdimage but 21 is empty. However pending/ still points to 20.