Improve validator error messages about cedar HOT 3 CLOSED

Error one can be improved without too much effort.

Errors two and four are already fixed (#137 and #132). For error two it still prints JSON, but this is desirable IMO because that's what the user types in the schema (unless we want to create that non-json schema format :) ).

Error three relates to a broader class of poor errors from our parser. Ideally we could improve all of them at once. Regarding line numbers for schema parsing errors. There's no clear way to capture line numbers when using serde to parse JSON into our schema "CST". Adding line numbers to schema errors would be a large rework of the schema parsing code.

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I looked into getting at the source locations a bit and found this: serde-rs/json#637

It definitely looks likes it's possible. There's a crate linked that does something like what we want. It's not quite exactly what we need, but the implementation shows how we could do this ourselves.

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Closing in favor of #174 #175 #176

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