Error handling guide

Now that we've done `fail` -> `panic`, I feel bringing back the error handling guide is a good idea. We had one long ago, but it was removed when conditions were removed.

This doesn't cover the new FromError stuff, but I feel like it's already useful in this state, so I'm sending this PR now.
Hm, maybe replace `0.95` with `// ...` as the implementation would normally be more sophisticated than this.