Return error instead of message from docker resource plan

[arichardet]

Instead of returning a message with the error, an error should be returned.

Sample output:

root/docker.network.test-network:
 Error: resource cannot change because of an error
 Messages:
  gateway 192.168.129.1 already in use
 Has Changes: yes
 Changes:
  test2: "absent" => "present"

Summary: 2 errors, 2 changes

 * root/docker.network.test-network: resource cannot change because of an error
 * root: error in dependency "root/docker.network.test-network"