Implement `core::error::Error` for `ErrorIterator`

[tgross35]

To do this ErrorIterator needs a Display impl, which could require making it a wrapper over Box<dyn Iterator<Item = ValidationError<'a>> + Sync + Send + 'a>, rather than just a type alias (a mild breaking change).

This will allow throwing this error to dyn Error or anyhow without any manual mapping. This impl should probably just print a list of all errors, like the examples show. Roughly:

type ErrorIterTy<'a> = Box<dyn Iterator<Item = ValidationError<'a>> + Sync + Send + 'a>;
pub struct ErrorIterator<'a>(ErrorIterTy<'a>);

impl<'a> IntoIterator for ErrorIterator<'a> {
    type Item = ValidationError<'a>;
    type IntoIter: ErrorIterTy<'a>;

    fn into_iter(self) -> Self::IntoIter {
        self.0
    }
}

impl fmt::Display for ErrorIterator {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
        writeln!(f, "Validation errors:");
        for (idx, e) in self.enumerate() {
            writeln!(f, "{idx:02}: {e}")?;
        }
        Ok(())
    }
}

impl core::error::Error for ErrorIterator {}