ofp-errors: Remove OFPERR_* values for error categories.

ofp-errors: Remove OFPERR_* values for error categories.

The main use of OFPERR_* is to define specific errors, but OFPERR_* also
existed for each possible category of error, to enable partial decoding of
unknown specific errors within a known category.  However, in practice,
it was very easy to misuse the error categories as if they were particular
errors.  This commit removes the error category values, to make that error
impossible.

(If partial decoding of unknown specific errors turns out to have been a
valuable feature, then we can reintroduce it some other way.)

Signed-off-by: Jarno Rajahalme <jarno.rajahalme@nsn.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>