Retain zbus errors on connection failures

[complexspaces]

Our current implementation of SecretService::connect is non-ideal as it throws away valuable failure information when it fails to connect:

secret-service-rs/src/util.rs

Lines 146 to 154 in f53e2e7

	pub(crate) fn handle_conn_error(e: zbus::Error) -> Error {
	match e {
	zbus::Error::InterfaceNotFound | zbus::Error::Address(_) => Error::Unavailable,
	zbus::Error::InputOutput(e) if e.kind() == std::io::ErrorKind::NotFound => {
	Error::Unavailable
	}
	e => e.into(),
	}
	}

It would be better if Error::Unavailable changed from a unit variant to hold the original zbus error. Its currently unclear if this can be done without a major breaking change.