Trickle up errors instead of directly logging

[Keksoj]

As discovered in this Pull Request:

• Session creation: register before creating them #824

Errors are too often logged in the middle of low-level functions, when they should rather get trickled up and logged by the logic calling them.

A good example is in command/src/channel.rs:

sozu/command/src/channel.rs

Lines 119 to 132 in 68e3859

	pub fn run(&mut self) {
	let interest = self.interest & self.readiness;
	if interest.is_readable() {
	let _ = self.readable().map_err(|e| {
	error!("error reading from channel: {:?}", e);
	});
	}
	if interest.is_writable() {
	let _ = self.writable().map_err(|e| {
	error!("error writing to channel: {:?}", e);
	});
	}

This should rather return an anyhow::Result<()> with context, to be handled downstream.

Feel free to comment below on where this change should be applied (or not applied), with github permalinks. Also, quote this issue in ongoing pull requests.

[Keksoj]

It seems enough for now. It has gotten too far, even. In the future, we may prefer to use custom Errors in each module and use thiserror instead of anyhow.

See #910