Fix or suppress clippy warnings

Clippy complained about functions that unnecessarily wrap their return value into Result even though they do not ever return an Err.

https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_wraps

In one place Clippy seems right and I followed its advice to remove the Result. The two other places actually should rather be changed to return errors. The code already has comments stating that and I attributed the code now to suppress the Clippy warnings.