Improve json cast error messages

[dnwpark]

Provide context to error messages when casting to from json to collections.

Given SELECT <tuple<a: int64>>to_json('{"b": 1}'); the following error is produced:

InvalidValueError: while casting 'std::json' to 'tuple<a: std::int64>', at tuple element 'a', missing value in JSON object

Given SELECT <tuple<a: int64>>to_json('{"a": "b"}'); the following error is produced:

InvalidValueError: while casting 'std::json' to 'tuple<a: std::int64>', at tuple element 'a', expected JSON number or null; got JSON string

[dnwpark]

Looks bigger than it is :)

[msullivan]

This looks great, and will be a nice improvement.

Remember to bump EDGEDB_CATALOG_VERSION

[msullivan]

I think it would be cleaner to build the dict and json.dumps it

[msullivan]

We want this because it produces a better error message than leaving it to the cast, or what?

[dnwpark]

This function lets us produce different error messages for when a tuple is missing an element and when the element is there but explicitly set to null.

[msullivan]

The big question for me with this PR is:
Is it better to have the context prepended to the actual message, or would it be better to put it in the details field?

My inclination had been details, but this looks pretty nice.

[dnwpark]

I guess either works, but it seemed to me that details were often more about providing information from beyond the immediate context. (eg. stuff about the inheritance of the types involved in an error)