Allow filter chains to start with a missing variable #496
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Following up on #477, this change treats missing variables and indexes as
nilwhen they appear at the start of a filter chain. As far as I can tell, this is in line with the shopify implementation.I needed a way to properly identify when an error identified a missing index or variable, and so introduced the concept of an
ErrorKindto handle this. If there is a better or preferred way, please let me know!closes #477