Issue explicit error messages for attempts to use "shell" operators in

ordinary expressions.  This probably doesn't catch every single case
where you might get "cache lookup failed for function 0" for use of a
shell operator, but it will catch most.  Per bug #4120 from Pedro Gimeno.

This patch incidentally folds make_op_expr() into its sole remaining
caller --- the alternative was to give it yet more arguments, which
didn't seem an improvement.