wic: Don't allow mkfs to fail silently in partition command
The return code from the mkfs command used by the partition creation
command was being ignored, allowing it to silently fail and leaving
users mystified as to why the resulting filesystem was corrupted.
This became obvious when failures occurred when creating large
e.g. sdk filesystems [YOCTO #6863].
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
