Fix handle NO [UNKNOWN-CTE] answer according to RFC 3516

http://trac.roundcube.net/ticket/1490046
According to RFC3516 "If the server does not know how to decode the section's CTE, it
         MUST fail the request and issue a "NO" response that contains
         the "UNKNOWN-CTE" extended response code."

This patch makes roundcube to try to fetch a message using "BODY" if the request fails using "BINARY" instead of showing "messageopenerror".