[SCSI] Make error printing more verbose

This patch enhances SCSI error printing by:

 - Making use of scsi_print_result() in the completion functions.

 - Having scmd_printk() output the disk name (when applicable).

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>