Previously, `E1!void` failed to coerce to `E2!E1!void` because it tried
to coerce the `E1` error to an `E2` if comptime-known and refused to
attempt any other coercion. This is a strange language rule with no
clear justification, and breaks real use cases (such as calling `getOne`
on an `Io.Queue(E!T)`). Instead, only error *sets* should try to coerce
to an "error" value of an error union type.