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This is a total feature request that might be worth ignoring... but when I've used other templating engines... if a variable isn't found, the default is to simply ignore the variable, rather than error out and prevent the rest of the template from rendering.
This is, of course, a completely opinion driven design thing, but I feel like it's better for the user (template author) if the rest of the template loads even if certain variables aren't found.
Not sure how to even begin to accomplish this (noob programmer...) but just a thought.