py: Add MP_OBJ_STOP_ITERATION and make good use of it.

Also make consistent use of MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL.
This helps a lot in debugging and understanding of function API.

py/objfloat.c

@@ -140,7 +140,7 @@ mp_obj_t mp_obj_float_binary_op(int op, mp_float_t lhs_val, mp_obj_t rhs_in) {
         case MP_BINARY_OP_MORE_EQUAL: return MP_BOOL(lhs_val >= rhs_val);
 
         default:
-            return MP_OBJ_NULL; // op not supported
+            return MP_OBJ_NOT_SUPPORTED;
     }
     return mp_obj_new_float(lhs_val);
 }