Hey, can someone give me a hint how this "bug" is possible in _embedded-hal-mock_ (eh1 spi mock)?

```
impl<W> spi::ErrorType for Mock<W>
where
    W: Copy + Debug + PartialEq,
{
    type Error = spi::ErrorKind;
}
```

Where the problem is that in `spi::ErrorType` the type is declared as:
```
type Error: Error
```
but `spi::ErrorKind` doesn't implement the `Error` trait?