Improve shared subcommand error

Co-authored-by: John Gee <[email protected]>

Author: aweebit

lib/command.js

@@ -271,8 +271,8 @@ class Command extends EventEmitter {
 - specify the name in Command constructor or using .name()`);
     }
     if (cmd.parent) {
-      throw new Error(`'${cmd._name}' cannot be added using .addCommand()
-- it already has a parent command`);
+      throw new Error(`Command '${cmd.name()}' passed to .addCommand() already has a parent
+- subcommands cannot be shared`);
     }
 
     opts = opts || {};