Fix exit code display for task failure

Fix exit code display for task failure

Per the python documentation, os.waitpid returns the exitcode shifted up by 8
bits, and we weren't compensating, resulting in a display of 'failed with 256'
when a worker process exits with a code of 1.

(Bitbake rev: 90c2b6cb24dc9c82f0a9aa9d23f2d1ed2e6ff301)

Signed-off-by: Chris Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>