scsi_debug: give unit attention and other errors precedence over TSF

Give existing errors priority over the generation of Task
Set Full (TSF) errors. So that max_queue is not exceeded,
existing errors may be sent back in the invocation thread.
This is done so errors like Unit Attentions are not hidden
and lost by either max_queue exceeded or real/injected
TSFs.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>