Remove lcb_error_handler. Invoke callback directly

We only use this in one place now, so remove the extra cruft in error.c

Change-Id: Iab0419e51cc8cb1230713d0eacae460ab1580d0d
Reviewed-on: http://review.couchbase.org/39383
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>