shared/shell: Print out help if using invalid command

Because Vincent Petry kindly reported that it is hard to use a command
in a submenu, clients are changed to print out help messages as below
when an invalid command is used.

  [bluetooth]# list-attributes
  Invalid command in menu main: list-attributes

  Use "help" for a list of available commands in a menu.
  Use "menu <submenu>" if you want to enter any submenu.
  Use "back" if you want to return to menu main.
1 file changed