Fix DockerErrorHandler’s use of Strings2.toString

jclouds-labs master is broken, e.g. see https://jclouds.ci.cloudbees.com/job/jclouds-labs-pull-requests/209/org.apache.jclouds.labs$docker/console which reports the error:

    [ERROR] /scratch/jenkins/workspace/jclouds-labs-pull-requests/docker/src/main/java/org/jclouds/docker/handlers/DockerErrorHandler.java:[95,25] toString() in java.lang.Object cannot be applied to (org.jclouds.io.Payload)

The problem is that the deprecated Strings2.toString has been deleted (in https://github.com/jclouds/jclouds/commit/fb60d76704745317186ad700e9b7ce5a569a614d).