installer: Make sure there is at least 1 target to be installed.

Ancient chroots did not provide a target list, this serves as a reminder.

Fixes #749 and #788.
Improve ze text.
test/16-targetsfile: Test if installer detects an absent or empty /etc/crouton/targets
Merge pull request #790 from drinkcat/update-error-no-target

installer: Make sure there is at least 1 target to be installed.
use ~/Downloads as home directory for chroot user