recover

This code just seems incorrect. As it stands today it reports a
successful restore if RecoverTask fails and then DestroyTask succeeds.

This can result in a really annoying bug where it then calls RecoverTask
again, whereby it will probably get ErrTaskNotFound and call DestroyTask
once more.

I think the only reason this has not been noticed so far is because most
drivers like Docker will return Success, then nomad will call
RecoverTask, get an error (not found) and call DestroyTask again, and
get a ErrTasksNotFound err.