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.