Unhelpful Display implementation for FloatIsNaN

```rust
extern crate ordered_float;

fn main() {
    println!("{}", ordered_float::NotNaN::new(::std::f64::NAN).unwrap_err());
}
```

```
FloatIsNan
```

This message (and Display output in general) should be designed to be human-readable and self explanatory. PascalCase belongs in Debug impls.