Add alternative solution using From trait

solutions/13_error_handling/errors6.rs

@@ -29,6 +29,21 @@ impl ParsePosNonzeroError {
     }
 }
 
+/// As an alternative solution, implementing the `From` trait allows for the
+/// automatic conversion from a `ParseIntError` into a `ParsePosNonzeroError`
+/// using the `?` operator, without the need to call `map_err`.
+///
+/// ```
+/// let x: i64 = s.parse()?;
+/// ```
+///
+/// Traits like `From` will be dealt with in later exercises.
+impl From<ParseIntError> for ParsePosNonzeroError {
+    fn from(err: ParseIntError) -> Self {
+        ParsePosNonzeroError::ParseInt(err)
+    }
+}
+
 #[derive(PartialEq, Debug)]
 struct PositiveNonzeroInteger(u64);