ofp-errors: Correctly encode errors as extensions or not depending on domain.

ofp-errors: Correctly encode errors as extensions or not depending on domain.

When ofp-errors was introduced, each OFPERR_* was either an extension or
not.  However, since then, some Nicira extension error code have been
given official error codes in later OpenFlow versions, so now whether an
OFPERR_* is an extension depends on the OpenFlow versions.  This means
that certain errors were encoded incorrectly as extensions in later
OpenFlow versions.  This commit fixes the problem.

This commit also adds a test that should prevent a regression.

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