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.
Create a symbolic link from /usr/local/chroots to /mnt/stateful_partition/crouton:
Makes sure that dev_install does not wipe the chroots.

Also, migrate existing chroots to new directory (transparent to the user, and very
fast as we stay on the same filesystem).
Fixes after review.
Install chroots in /mnt/stateful_partition/crouton/chroots
so we can put other stuff in /mnt/stateful_partition/crouton (e.g. shared directories).