"Upgrade from ISO" doesn't error on fresh disk

When running the Tails installer - “Upgrade from ISO” appears to upgrade successfully on a disk that is actually blank. This should produce an error and say something to the effect of “the disk you have selected does not already have Tails installed, we cannot upgrade it” or it should properly install Tails. It would make sense to make another button for properly installing Tails but that seems like another issue.

Related issues

Related to #5623 (closed)

Original created by @ioerror on 7659 (Redmine)

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