(PUP-2882) Fix single available upgrade

Prior to this commit `puppet module upgrade` would fail with an error message
about not being able to satisfy the upgrade when there is only one published
release of the module.
This commit changes the error message to say that there is no version to upgrade
to. This should reduce some user confusion.
Merge pull request #2885 from highb/fix/master/pup-2882_upgrade_broken_for_one_version

(PUP-2882) Fix single available upgrade