wipefs: exit on failed erase

wipefs: exit on failed erase

The current behavior is to report error and continue, it seems strange:

	# blockdev --setro /dev/sdc

	# wipefs -a /dev/sdc
	wipefs: /dev/sdc: failed to erase xfs magic string at offset 0x00000000: Operation not permitted
	/dev/sdc: 4 bytes were erased at offset 0x00000000 (xfs): 58 46 53 42
                          ^^^^^^^^^^^
                           not true

The patch calls err() to exit.

Reported-by: Vratislav Podzimek <vpodzime@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>