Add JSON output for errors

There is already an --emit json option that emits formatting changes as JSON. However if Rustfmt encounters a parse error it prints the error to stderr in human readable form and exits with error code 1.

$ rustfmt --emit json lib.rs
error: expected one of `!` or `::`, found `<eof>`
  --> /.../src/lib.rs:16:1
   |
16 | sdf
   | ^^^ expected one of `!` or `::`

failed to parse Real("/.../src/lib.rs")

It would be helpful in some situations to be able to have these errors in JSON format too. For example for IDE integration or CI. (One of my coworkers is using a horrible regex to parse this, against my advice!)

I think an --emit-errors json option would be reasonable.

• rustfmt version: rustfmt 2.0.0-rc.2-nightly (e91edf56 2020-08-04)
• From where did you install rustfmt?: Git master, from yesterday.

Start by tracing the existing --emit json handling and the parse-error path that currently writes human-readable diagnostics to stderr. Determine the JSON error shape and how an --emit-errors json option should behave; done means parse errors are emitted in JSON while preserving the documented failure status.