Fix error message for unintialized partition [Closes: 857]

Fix error message for unintialized partition [Closes: 857]

Do an explicit check if specified partition is at least readable and do
not use blkid for it as blkid uses returnvalue 2 for the same reason (no
information availabe, wrong permissions). This will give better feedback
to the user, e.g. if he forget to format the partition.