ENDP optional 2nd arg

[MOON at SCRC-TENEX]

In our debugger, where the arguments to functions are always available,
and the arguments to the function that err'ed are displayed as part of
the initial error message, the extra argument to ENDP would be superfluous.
I think this is a better approach since it handles the problem generally
rather than handling one specific case that someone happened to think of
first.