Common prefix

Let's say I want to have a common prefix to all my errors, like:

```rust
#[derive(Error)]
enum MatrixParseError {
#[error("Couldn't parse matrix: foo")]
Foo
#[error("Couldn't parse matrix: bar")]
Bar
}
```

It would be cool to have the ability to specify such a prefix. (E.g. as `[error("Couldn't parse matrix: {}")]` on the enum.)

Do you think you could add that? Or do you recommend another idiomatic way of achieving this behavior?