rustfmt should display errors

If you run rustfmt and it errors out, it currently just "beeps" and suggests you switch over to *rustfmt*. I think this should be treated like a failed compilation; we should split the frame into two windows, show the errors, and let you use M-x next-error to view them.

Example 1, which errors because it doesn't parse:

fn main() {
    let x &'static str = "aaa";
}

Example 2, undefined module:

mod a;

fn main() {
    let x: &'static str = "aaa";
}

Example 3, long lines:

fn main() {
    let x:
    &'static str = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
}

Start by tracing how rustfmt errors are currently reported and how the rustfmt buffer is opened. Compare the requested behavior with Emacs's M-x next-error workflow, using the three Rust examples to check parse errors, undefined modules, and long-line errors. Done means errors are shown in a split frame and can be navigated with M-x next-error.