ofp-errors: Send as much of a message as possible in an error reply.

ofp-errors: Send as much of a message as possible in an error reply.

When an OpenFlow switch sends an error message in reply to a request
from a controller, it includes an excerpt from the original request.  The
OpenFlow specification only requires the switch to send at least the first
64 bytes of the request.  Until now, Open vSwitch has only sent that much.
This commit changes Open vSwitch to instead send the entire message, only
trimming it if necessary to keep the error message within the 64 kB limit
for an OpenFlow message.  This should simplify debugging for controller
authors.

CC: Suneel Bomminayuni <sbomminayuni@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>