Add error codes for Open Flow v1.2

Add error codes for Open Flow v1.2

* Where Open Flow 1.2 breaks apart error codes defined
  in previous versions, provide all new definitions to
  previous versions and map the numeric error code to
  the first first definition supplied in ofp-errors.h.
  The case handled so far is:
  OFPERR_OFPBIC_BAD_EXP_TYPE -> { OFPERR_OFPBIC_BAD_EXPERIMENTER,
                                  OFPERR_OFPBIC_BAD_EXP_TYPE }

* Where Open Flow 1.2 adds error codes that were previously
  defined as Nicira extension errors define the later in terms
  of the new codes.

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com added better error checking in extract-ofp-errors, added
 unit tests, miscellaneous cleanup]
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Simon Horman <horms@verge.net.au>