address review feedback

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.

exercises/error_handling/errors5.rs

@@ -43,8 +43,8 @@ impl PositiveNonzeroInteger {
 impl fmt::Display for CreationError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let description = match *self {
-            CreationError::Negative => "Number is negative",
-            CreationError::Zero => "Number is zero",
+            CreationError::Negative => "number is negative",
+            CreationError::Zero => "number is zero",
         };
         f.write_str(description)
     }