Modified "chewtoy".
1. Replaced text of whiptail menu's "cancel" button with "Quit".
2. Replaced EXIT trap within get_selection_via_whiptail() with a trap
   on RETURN.  Traps set within functions are local to the function,
   so an EXIT trap in that context will never get called.
3. Modified get_selection_via_whiptail() to handle whiptail failures
   (non-zero exit values).  Now, if whiptail fails, a plain menu will
   be displayed via get_selection().