[select] Add LCB_ERROR_EVENT

This special event is delivered when an FD is set with 'exceptfds'. In
this case we check the socket error with getsockopt, and hopefully be
more timely when a connection is refused.

Change-Id: I96865df6800e10add25244029fc34cb85b8a7730
Reviewed-on: http://review.couchbase.org/28134
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>