Add an example of collecting errors while iterating successes

Inspired by the comments from people looking for something like this in <rust-lang/rust#87047 (comment)>.

src/error/iter_result.md

@@ -30,6 +30,25 @@ fn main() {
 }
 ```
 
+## Collect the failed items with `map_err()` and `filter_map()`
+
+`map_err` calls a function with the error, so by adding that to the previous
+`filter_map` solution we can save them off to the side while iterating.
+
+```rust,editable
+fn main() {
+    let strings = vec!["42", "tofu", "93", "999", "18"];
+    let mut errors = vec![];
+    let numbers: Vec<_> = strings
+        .into_iter()
+        .map(|s| s.parse::<u8>())
+        .filter_map(|r| r.map_err(|e| errors.push(e)).ok())
+        .collect();
+    println!("Numbers: {:?}", numbers);
+    println!("Errors: {:?}", errors);
+}
+```
+
 ## Fail the entire operation with `collect()`
 
 `Result` implements `FromIter` so that a vector of results (`Vec<Result<T, E>>`)