py: Fix location of VM returned exception in invalid opcode and comments

The location for a returned exception was changed to state[0] in
d95947b48a

py/objfun.c

@@ -323,7 +323,7 @@ STATIC mp_obj_t fun_bc_call(mp_obj_t self_in, size_t n_args, size_t n_kw, const
     } else {
         // must be an exception because normal functions can't yield
         assert(vm_return_kind == MP_VM_RETURN_EXCEPTION);
-        // return value is in fastn[0]==state[n_state - 1]
+        // returned exception is in state[0]
         result = code_state->state[0];
     }