libtime: `strftime` now returns `Result<String, String>` instead of `String`.



[breaking-change]
>  Also, do you know of a concrete case in which this can actually fail?

Nope, just felt it was safer than calling `unwrap`. I did not check what cases `from_utf8` may fail. Isn't it just safer and more future proof to do it this way? It's not really a lot of code.