CC should raise error when no DEA available to run a APP instance.

Now `cf push` will get 'OK' response even when there's no DEA available to run a user's APP. This will cause 0% health or 50% on cf's side without any clue for end user.

I think it's better to raise error in CC in order to tell the user what's wrong, rather than log the error silently inside the system.

Please add the additional expectation setup so that `DeaClient.start(app)` does not raise an error. (See the above examples in this context for how to do this.)