wic: Continue if excluded_path does not exist

wic: Continue if excluded_path does not exist

If an excuded path does not exist, continue without an error.
This allows to seamleasly reuse .wks among different projects.

Eg:

part / --source rootfs --fstype=ext4 --exclude-path=opt/private_keys

Where /opt/private_keys in only populated by some of the image.bb files.

Cc: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>