Add ecode to rpc.py's RpcResult.Raise()

This patch adds a new ecode argument to RpcResult.Raise(). This allows
specifying the error code (for both OpExec and OpPrereq errors).

Note that this patch also makes the OpExecError exceptions raised from
_FindFaultInstanceDisks have the error code classification.

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