Return errors from parsing inner items

[djc]

Thanks to @doyshinda for pointing this out in #585.

Fixes #587.

[valkum]

The tests currently fail because in test_filtered_scanning HSCAN actually returns a (String, usize), the field and the value.

Changing

let iter = con.hscan_match("foo", "key_0_*").unwrap();

for x in iter {
        // type inference limitations
        let x: usize = x;
        unseen.remove(&x);
    }

to

let iter = con
        .hscan_match::<&str, &str, (String, usize)>("foo", "key_0_*")
        .unwrap();

    for (_, x) in iter {
        unseen.remove(&x);
    }```

fixes the test.

[djc]

(CI is not running due to the recent branch rename, see #612.)