wic: fix WicError message

* add missing % to print the values instead of:
  | INFO: Build artifacts not found, exiting.
  | INFO:   (Please check that the build artifacts for the machine
  | INFO:    selected in local.conf actually exist and that they
  | INFO:    are the correct artifacts for the image (.wks file)).
  |
  | ERROR: ("The artifact that couldn't be found was %s:\n  %s", 'kernel-dir', '/OE/build/deploy/images/qemux86-64')

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e104c2b1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>