der: Move `error.position` from Error to Reader

[dishmaker]

Currently, it is annoying to track error.position in every function returning der::Error.

From another issue:

Attempting to even construct der::Error was confusing [...] It is also unclear to me whether the position that this method uses will be correct [...]

• der: Document (or improve) der::Decode trait impl error handling #1492

formats/der/src/error.rs

Lines 23 to 24 in 9813667

	/// Position inside of message where error occurred.
	position: Option<Length>,

Actually, most functions don't return the position - example:

formats/der/src/datetime.rs

Lines 79 to 83 in 9813667

	|| !(0..=23).contains(&hour)
	|| !(0..=59).contains(&minutes)
	|| !(0..=59).contains(&seconds)
	{
	return Err(ErrorKind::DateTime.into());

What I suggest is something like serde + quick-xml:

let mut reader = SliceReader::new(EXAMPLE_MSG).unwrap();
let err = SomeStruct::decode(&mut reader);
assert!(err.is_err());

assert_eq!(reader.error_position(), 28);

https://docs.rs/quick-xml/latest/quick_xml/de/struct.Deserializer.html#method.get_ref

tafia/quick-xml#743