Improve error handling guide

[ealmloff]

Error Handling Guide

The error handling guide was partially updated for 0.6, but some sections still reference Option<VNode>. We need to update the guide and potentially move it to the essentials section. There are also a few improvements we can make to the error handling guide:

• The first two sections return errors with ? without introducing the error type
• Throwing an error from event handlers is not mentioned in the guide
• Error handling in the result of resources isn't mentioned in the guide, but can be difficult when the error type is not clone (like reqwest::Error)
• The guide should talk about the context and show methods on the Context trait