Fix setup-storage handling of empty disk(s) with parted >=3.*

When executing 'parted -s /dev/sda unit TiB print' on an empty
disk with parted 2.3(-16) it exits with return code 1 and
the following line on stdout:

| Error: /dev/sda: unrecognised disk label

When executing the same command with parted 3.* (verified with
3.2-5 as present in current Debian jessie/sid) it also exits with
return code 1, but outputs the error message on std*err*.

Without this patch executing setup-storage on an empty disk
fails with:

| Executing: parted -s /dev/sda unit TiB print
| (STDERR) Error: /dev/sda: unrecognised disk label
| (STDOUT) Model: ...
| (STDOUT) Disk /dev/sda: ...
| (STDOUT) Sector size (logical/physical): 512B/512B
| (STDOUT) Partition Table: unknown
| (STDOUT) Disk Flags:
| Command had non-zero exit code