Add 'OPFAIL' command

Allows forcing specific error codes

Change-Id: I2e1bfeafdc68094e914076aa28f541492784b0ba
Reviewed-on: http://review.couchbase.org/29989
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>