If, indeed, yours was, as you put it, a "fool's errand," it is one that is and has been shared by many, evidently! As mentioned above, the statement is something of an "urban legend," and does come up un discussion from time to time. At least this all points to the lasting memory of Mantan Moreland.
call me crazy, but I thought he used a phrase like that in one of the Winters Chan films. It's the one film where they visit a funeral parlor at night. It had Milton Parsons in it as a character named Grail. Or maybe the phrase is "I'll tell you when to go". And didn't Willie Best make a similar comment in one of the Toler movies?