[SCSI] storvsc: Restructure error handling code on command completion

In preparation for handling additional sense codes, restructure and cleanup
the error handling code in the command completion code path.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>