It reports LCB_DLOPEN_FAILED if built without IO plugins

Change-Id: I8f17b2bf748fc29c3413194ec4968f6be4131f3a
Reviewed-on: http://review.couchbase.org/26114
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>