container: add ErrNoSuchContainer

This error eases error handling. Clients of the package will be able to
tell invalid IDs from server errors.
image: add ErrNoSuchImage variable

This variable will be used to identify failure in image requests, and
container creation.
container: return ErrNoSuchImage on CreateContainer when 404 is returned
container: use variables instead of magic numbers
container: return proper error in StartContainer
container: return proper error on StopContainer
container: return proper error on RestartContainer
container: return proper error in KillContainer
container: return proper error in RemoveContainer
container: return proper error on WaitContainer