Introduce two-argument style for OpPrereqError

This patch introduces a two-argument style for OpPrereqError. Only the
direct raise calls in cmdlib.py are converted, other users will follow.

cli.py is modified to handle both two-argument style and the current
format. RAPI doesn't need modification as the way we encode errors is
already using a list for the error arguments, so RAPI users only need to
start checking the list length and the second argument.

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