wic: _exec_cmd: produce error if exit code is not 0
Current code doesn't always show error output of the
external command and even ignores non-zero exit code.
Moved checking of exit code value to the lowest level
possible: to _exec_cmd. This should make wic to always
check exit code of the external command and issue
an error if it's not 0.
[YOCTO #10816]
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
