error_handling/errorsn.rs uses deprecated Trait - related syntax

[sosnowski]

Hi!

I've noticed, that the exercise:

/exercises/error_handling/errorsn.rs is using a deprecated syntax for traits related values.
It causes the compiler to display warning messages:

rustlings run exercises/error_handling/errorsn.rs
! Compiling of exercises/error_handling/errorsn.rs failed! Please try again. Here's the output:
warning: trait objects without an explicit `dyn` are deprecated
  --> exercises/error_handling/errorsn.rs:23:30
   |
23 | fn read_and_validate(b: &mut io::BufRead) -> Result<PositiveNonzeroInteger, Box<error::Error>> {
   |                              ^^^^^^^^^^^ help: use `dyn`: `dyn io::BufRead`
   |
   = note: #[warn(bare_trait_objects)] on by default

warning: trait objects without an explicit `dyn` are deprecated
  --> exercises/error_handling/errorsn.rs:23:81
   |
23 | fn read_and_validate(b: &mut io::BufRead) -> Result<PositiveNonzeroInteger, Box<error::Error>> {
   |                                                                                 ^^^^^^^^^^^^ help: use `dyn`: `dyn error::Error`

warning: trait objects without an explicit `dyn` are deprecated
  --> exercises/error_handling/errorsn.rs:32:65
   |
32 | fn test_with_str(s: &str) -> Result<PositiveNonzeroInteger, Box<error::Error>> {
   |                                                                 ^^^^^^^^^^^^ help: use `dyn`: `dyn error::Error`

warning: trait objects without an explicit `dyn` are deprecated
   --> exercises/error_handling/errorsn.rs:101:31
    |
101 |         f.write_str((self as &error::Error).description())
    |                               ^^^^^^^^^^^^ help: use `dyn`: `dyn error::Error`

Probably the solution is as simple as changing Box<Trait> to a recomended Box<dyn Trait> (or maybe impl Trait is better?

I will prepare a simple pull request with a fix.
I'm fairly new to Rust, so if what I'm saying does not make sense, just let me know ;)

rustc 1.37.0 (eae3437df 2019-08-13)
rustlings 1.4.1