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.
perro tuerto
mfurak