Fix the issue that when docker exec a paused container, it will always

I think we should check the container status whether is paused  before exec a container. and it should not exec a paused container.

Signed-off-by: Chen Chao cc272309126@gmail.com
I don't think this fatal is correct because err is nil.  You should have something like:
`t.Fatal("container should fail to exec new command")`