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.
Improve the hints for move_semantics5, as well as the explanatory
comments in the code.
Previously, it was not clear what possible changes were allowed.
It seems that reordering the statements might be the intended solution.
The previous comment about not "adding newlines" doesn't make sense,
so treating it as "adding new lines" makes it more clear.
Add hints about how to return the correct type for functions that
return `Result<_, Box<dyn Error>`. Some feedback from Discord suggests
that people run into trouble with that.
Use `Box<dyn error::Error>` to allow solutions to use `?` to propagate
errors. In the tests, explicitly check `is_ok()` instead of trying to
force the error type to `String` (or other `PartialEq` type) using
`assert_eq!()`.
Taylor Yu