[SCSI] set error value when failing commands in prep_fn

set DID_NO_CONNECT for the BLKPREP_KILL case and correct a few
BLKPREP_DEFER cases that weren't checking for the need to plug the
queue.

Signed-Off-By: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>