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;

Here's the error in Rust 1.11: 6:11 error: cannot assign to `x` because it is borrowed [E0506]     x += 1;
             ^~~~~~ 4:15 note: borrow of `x` occurs here     let y = &x;
                      ^ 6:11 help: run `rustc --explain E0506` to see a detailed explanation

Here's the error in Rust 1.28:

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

error: aborting due to previous error

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.