wic: Make exec_cmd() error out instead of warn

wic: Make exec_cmd() error out instead of warn

The reason exec_cmd() warns but doesn't error out (broken parted)
doesn't really make sense, since the parted invocations don't even use
exec_cmd().  It really should just fail since by not doing so it's
actually enabling invalid images in some cases.

Also, since the return code is now always zero, there's no point in
having a return code, so remove it.  This represents a change in the
API, so we also need to update all callers.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>