Remove exceptions from network code

[glebm]

Network code is the only place in the codebase where we use exceptions.

Removing exceptions from network code will pave a path for network support on platforms that do not support exceptions (e.g. nxdk). I also think it will make the code easier to follow.

Exceptions can be replaced with tl::expected.

ASIO can be configured to avoid exceptions, we just need to provide a custom "throw" implementation. More information from @StephenCWills:

Found some info for building ASIO without exceptions.
chriskohlhoff/asio#302 (comment)

Here's an option I thought of based on your work in this PR. Something like...

tl::expected<void, PacketError> tcp_client::poll()
{
	while (ioc.poll_one() > 0) {
		if (ioHandlerError != nullptr)
			return t1::make_unexpected(*ioHandlerError);
	}
}