rollup merge of #18903: steveklabnik/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.

diff --cc src/libcore/result.rs
Simple merge