Allow filter chains to start with a missing variable

[rydrman]

• [x] Tests created for any new feature or regression tests for bugfixes.

Following up on #477, this change treats missing variables and indexes as nil when 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 ErrorKind to handle this. If there is a better or preferred way, please let me know!

closes #477

[rydrman]

I'm not sure why the AppVeyor build failed, it seems to be something outside of the scope of this PR

[jsoverson]

@epage any thoughts on this?

[jesseditson]

I'd love to see this land, but it looks pretty stale - reading https://github.com/cobalt-org/liquid-rust/issues/477 it's unclear to me - is the resolution that this should or should not be the correct behavior?