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.

Related PR of vendor/errors: https://github.com/cloudfoundry/errors/pull/15
Seems Travis CI can not be passed unless reference vendor/errors to my fork ...