Add an Other variant to VdafError

[divergentdave]

Closes #424. This implements the suggestion from that issue, adding an Other variant that accepts a boxed error trait object.

[cjpatton]

Wow, this literally came up yesterday in Daphne. I'm so for it.

[inahga]

LGTM, expected usage looks good.

    #[derive(Debug, thiserror::Error)]
    pub enum InahgaError {
        #[error("inahga error")]
        Oops,
    }

    #[test]
    fn test() {
        dbg!(VdafError::Other(Box::new(InahgaError::Oops)));
    }