[1.0] Restructure `PatternError`

Strongly typed errors would be helpful:

```rust
#[non_exhaustive]
pub struct PatternError {
    pub pos: usize,
    pub msg: PattternErrorKind,
}

#[non_exhaustive]
enum PattternErrorKind {
    // ...
}
```

This would make it easier for any crates using `glob` to transform errors into their own messages if desired.