Add chapter on error handling

[alice-i-cecile]

I've opted to cover this from a fairly elementary level: focusing on best practices within both Rust and programming for handling errors.

Bevy has some specific tools for helping with error handling, but in practice, most new Bevy users need to be told about and master the fundamentals before those are of much use.

To do

• error handling for systems: the Error type, system piping, the default error handler
• error handling for commands: why this is hard, queue_handled and the try_ variants
• configuring the default error handler

[alice-i-cecile]

This needs a section where we actually explain what defensive programming is and why it's good.