Rust 1.12 - new error format enabled

[sophiajt]

I just wanted to send out a friendly reminder that we're 17 days away from the release of Rust 1.12. With Rust 1.12, we'll be switching to the new error format (and removing the old format). There's also a standard JSON error format you can use.

I know @nikomatsakis has been watching this closely in emacs, since he and @wycats worked with me on the new format :)

Related: #156