Change 'fail!' to 'panic!' in text and code

In accordance with https://github.com/rust-lang/rfcs/pull/221. This will prevent confusion for new Rust developers who follow the book and get compile errors when 'fail!' is used :confused: .

I left the 'Fail!' text in here because it seemed to be a description of what was happening, rather than a command:
```rust
#[test]
fn this_tests_code() {
    panic!("Fail!");
}
```