misleading error message that prompts non-existing command

    % rails plugin new
    Options should be given after the plugin name. For details run: rails plugin --help
    
    % rails plugin --help
    /Users/a_matsuda/src/rails/activesupport/lib/active_support/dependencies.rb:228:in `require': cannot load such file -- rails/commands/plugin (LoadError)
    	from /Users/a_matsuda/src/rails/activesupport/lib/active_support/dependencies.rb:228:in `block in require'
    	from /Users/a_matsuda/src/rails/activesupport/lib/active_support/dependencies.rb:213:in `load_dependency'
    	from /Users/a_matsuda/src/rails/activesupport/lib/active_support/dependencies.rb:228:in `require'
    	from /Users/a_matsuda/src/rails/railties/lib/rails/commands.rb:51:in `<top (required)>'
    	from script/rails:6:in `require'
    	from script/rails:6:in `<main>'
    
    % rails plugin new --help
    Usage:
      rails plugin new APP_PATH [options]
    
    Options:
    ...