Improved error messages

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

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

1. fn main() {
2. let mut x = 5;
3. let y = &x;
4. x += 1;
5. println!("{} {}", x, y);
6. }

Here's the error in Rust 1.11:

1. foo.rs:4:5: 4:11 error: cannot assign to `x` because it is borrowed [E0506]
2. foo.rs:4     x += 1;
3. ^~~~~~
4. foo.rs:3:14: 3:15 note: borrow of `x` occurs here
5. foo.rs:3     let y = &x;
6. ^
7. foo.rs:4:5: 4:11 help: run `rustc --explain E0506` to see a detailed explanation
8. error: aborting due to previous error

Here's the error in Rust 1.28:

1. error[E0506]: cannot assign to `x` because it is borrowed
2. --> foo.rs:4:5
3. |
4. 3 |     let y = &x;
5. |              - borrow of `x` occurs here
6. 4 |     x += 1;
7. |     ^^^^^^ assignment to borrowed `x` occurs here

9. error: aborting due to previous error

11. 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 showsoff your code in context, rather than just showing the text of the lines themselves.