Rollup merge of #44466 - clarcharr:cow_error, r=alexcrichton

Add Cow<str> -> Box<Error> impls.

Considering how impls exist for `String` and `&str`, it makes sense to also add an impl for `Cow<str>` as well.

This would allow converting `String::from_utf8_lossy` directly into a `Box<Error>` or `io::Error` without having to add an extra `into_ownd()`.