[Patch] Emit error for undefined operands

[Frank Ch. Eigler]

Hi -

bje's second patch is fine, except that:

:   (let ((operands (map (lambda (x) (obj:name x)) (current-op-list)))
should be
#   (let ((operands (map obj:name (current-op-list)))

(The identity lambda function doesn't contribute much.)

- FChE
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