ofp-errors: Make every error encodable.

ofp-errors: Make every error encodable.

Until now, some values could not be encoded to send over OpenFlow, for a
few possible reasons.  This meant that, when one of these situations came
up, that a controller would not receive any notification that its request
failed.  This is not a good way to behave, so this commit changes the
error encoder so that, if a particular error cannot be encoded, it will
instead encode a fallback error code.

This commit also slightly simplifies ofconn_send_error() because it no
longer has to handle ofperr_encode_error() returning NULL.

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