Adjust error text and naming to conform with best practices.
Use `map_err()` instead of `or()`. Wrap lower-level errors instead of
ignoring their details.
Also, don't "cheat" by bypassing the `new()` function in tests.
Fix a dangling reference in the try_from_into hints.
Add new exercises errors5 and errors6, to introduce boxed errors and
custom error enums more gently. Delete errorsn, because it tried to do
too much too soon.
