Better abstract OpenFlow error codes.

Better abstract OpenFlow error codes.

This commit switches from using the actual protocol values of error codes
internally in Open vSwitch, to using abstract values that are translated to
and from protocol values at message parsing and serialization time.  I
believe that this makes the code easier to read and to write.

This is also one step along the way toward OpenFlow 1.1 support because
OpenFlow 1.1 renumbered a bunch of error codes.

Signed-off-by: Ben Pfaff <blp@nicira.com>