Fix error message

src/docker.js

@@ -122,7 +122,7 @@ function pullImage(manifest, callback) {
     // Use docker CLI here to support downloading of private repos. for dockerode, we have to use
     // https://github.com/apocas/dockerode#pull-from-private-repos
     docker.pull(manifest.dockerImage, function (error, stream) {
-        if (error) return callback(new DockerError(DockerError.EXTERNAL_ERROR, 'Error connecting to docker. statusCode: ' + error.statusCode));
+        if (error) return callback(new DockerError(DockerError.EXTERNAL_ERROR, 'Unable to pull image. statusCode: ' + error.statusCode));
 
         // https://github.com/dotcloud/docker/issues/1074 says each status message
         // is emitted as a chunk