Encode the actual exception raised by LU execution

Currently, the actual exception raised during an LU execution (one of
OpPrereqError, OpExecError, HooksError, etc.) is lost because the
jqueue.py code simply sets that to a str(err), and the code in cli.py
simply passes that string to OpExecError.

This patch moves to encoding the errors as per errors.EncodeError and
changes the cli code to parse and raise that (if possible).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>