Preserve $ref history.

[atsuoishimoto]

In large schemas, understanding which $ref is being referenced when an error occurs can be helpful in debugging. We used to refer to the history of $ref with code like this:

for error in validator.iter_errors(instance):
    print(validator.resolver._scopes_stack)
    print(error)

However, with the current version of jsonschema, validator.resolver is deprecated, and it will be not possible to refer to it.

In this PR, I have made changes so that the history of $ref is recorded in the exception object when an error occurs, and the $ref from the root to the part where the error occurred can be referred to with error.refs. I believe this feature will be useful not only for us, but for many users as well.

[Julian]

Your original code clearly touched something private :)

But for this one, I'm willing to consider such a thing, especially given it's required to support #1008, but not precisely in his way.

We'd also need to handle $dynamicRef and $recursiveRef, so some extra thought is needed.

But the objects used within the referencing library were written partially to support this (referencing's Resolver.dynamic_scope essentially contains this information).

I'm going to close this PR given the above but please open an issue and we can discuss some other way to do this.

[atsuoishimoto]

Thank you for the comment!

I filed the #1146. Thanks!