Add friendly error messages for IO errors

[ruuda]

Follow-up to this comment:

Currently we use io::Result in most places. These errors are not user-friendly, because they lose context about the file that we had a problem with (if it’s a file at all).

Also, all propagated errors get “handled” in main like so:

fastsync/src/main.rs

Lines 82 to 91 in cfba4cc

	Some("send") if args.len() >= 3 => {
	let addr = &args[1];
	let fnames = &args[2..];
	main_send(addr, fnames).expect("Failed to send.");
	}
	Some("recv") if args.len() == 3 => {
	let addr = &args[1];
	let n_conn = &args[2];
	main_recv(addr, n_conn).expect("Failed to receive.");
	}

I think it’s okay to crash the application in case of an IO error — I don’t think we need to be robust against transient failures, the user can just restart the transfer instead — but we can report clearly what went wrong and where.