check task existence before new task
check task existence before new task

make user know correct error when new task.

Signed-off-by: Ace-Tang <a967bc6ab01ace27203ca6ab9fd77fbd3b598184@126.com>

vdemeester/containerd,miaoyq/containerd,miaoyq/containerd,containerd/containerd,dmcgowan/containerd,dmcgowan/containerd,thaJeztah/containerd,thaJeztah/containerd,vdemeester/containerd,mikebrow/containerd,estesp/containerd,containerd/containerd,mikebrow/containerd,crosbymichael/containerd,estesp/containerd,crosbymichael/containerd