Move the luxi error handling into errors.py

Currently the luxi error handling is hardcoded as special encoding on
the masterd-side and special decoding on the client side. This patch
moves it to errors.py such that other parts of the code can reuse the
same encoding.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
(cherry picked from commit 6956e9cd)