Add support for error-format=json

[Popog]

The new error format broke all the addMarker calls for gitbook-plugin-rust-playpen. I'm trying to restore that functionality, and this seems like a good first step.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

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[alexcrichton]

Thanks!