add human-readable error when unknown pagespec function is called

It used to replace unknown functions with "0" when translating a pagespec.
Instead, replace it with a FailReason object. This way, the pagespec will
still evaluate as before (possibly successfully if other terminals exist),
but a human-readable error will be shown if the result is displayed.

Also, an empty pagespec used to be replaced with "0", to avoid a eval
error. Also use a FailReason here.

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