Allow errors in message parsing

[davidsteiner]

Currently, the inbound message parsing trait doesn't allow for errors to happen:

hotfix/crates/hotfix/src/message.rs

Lines 27 to 29 in 8b46ed0

	pub trait InboundMessage: Clone + Send + 'static {
	fn parse(message: &Message) -> Self;
	}

This isn't always realistic. We should allow implementors to returns errors.