wic: Fix confusing error message

Wic throws this message when any of the build artifacts are
not provided:
  Build artifacts not completely specified, exiting.
    (Use 'wic -e' or 'wic -r -b -k -n' to specify artifacts)
It was not clear which artifact was not specified.

Reworked the code to specify list of missed artifacts.
Now the message looks like this:
  The following build artifacts are not specified:
    bootimg-dir, kernel-dir, native-sysroot

[YOCTO #7912]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>