This code block was getting awfully complicated and did not work in my shell
configuration, so let's instead delegate the most complex logic and error
handling to the existing wrap_tails_create_iuks Python script (used by the
build_IUKs Jenkins job) and keep only very simple shell code in here.

Incidentally, this has an extra advantage: wrap_tails_create_iuks will download
any ISO that it can't find locally, which can be useful. Our doc asks the RM to
download such images at the beginning of their shift, but it's better not to
rely on this:

 - We sometimes split the release process in 2+ pieces; whoever handles
   this part might not be the one who did the earlier steps.

 - Between the beginning of a release cycle and release time, the RM might have
   deleted old images to save disk space locally.