Return with custom ErrorKind at nom 5.0

[NamsooCho]

Nom 5 don't have custom ErrorKind now.
But I want to return with custom error kind in this situation.

tox/src/toxcore/messenger/packet/msi.rs

Lines 237 to 246 in a8d0e56

	let request = if let Some(request) = request {
	request
	} else {
	return Err(Err::Error((input, ErrorKind::NoneOf)));
	};
	let capabilities = if let Some(capabilities) = capabilities {
	capabilities
	} else {
	return Err(Err::Error((input, ErrorKind::NoneOf)));
	};

It is needed when the Msi packet don't have a Request subpacket or Capability subpacket,
Now we return with ErrorKind::NoneOf, but it would be better to return with exact explanation why the error occurred.

The error kinds I want to use are

const NOM_CUSTOM_ERR_REQUEST_SUBPACKET_OMITTED: u32 = 1;
const NOM_CUSTOM_ERR_CAPABILITIES_SUBPACKET_OMITTED: u32 = 2;

Any idea?