Improved error messages

Minimum Rust version: 1.12

We're always working on error improvements, and there are little improvements in almost every Rust version, but in Rust 1.12, a significant overhaul of the error message system was created.

For example, here's some code that produces an error:

fn main() {
    let mut x = 5;
    let y = &x;
    x += 1;
    println!("{} {}", x, y);
}

Here's the error in Rust 1.11:

foo.rs:4:5: 4:11 error: cannot assign to `x` because it is borrowed [E0506]
foo.rs:4     x += 1;
             ^~~~~~
foo.rs:3:14: 3:15 note: borrow of `x` occurs here
foo.rs:3     let y = &x;
                      ^
foo.rs:4:5: 4:11 help: run `rustc --explain E0506` to see a detailed explanation
error: aborting due to previous error

Here's the error in Rust 1.28:

error[E0506]: cannot assign to `x` because it is borrowed
 --> foo.rs:4:5
  |
3 |     let y = &x;
  |              - borrow of `x` occurs here
4 |     x += 1;
  |     ^^^^^^ assignment to borrowed `x` occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0506`.

This error isn't terribly different, but shows off how the format has changed. It shows off your code in context, rather than just showing the text of the lines themselves.