fix when multi_node false.

Fixes problem described in
#1

lib/capistrano/node.rb

@@ -67,7 +67,7 @@ def remote_versions(path, prefix)
 
     desc 'Symlink node version'
     task :symlink_node do
-      return unless multi_node
+      exit unless multi_node
 
       unless node_version
         puts red('No suitable node version found')