Provide context when `?` can't be called because of `Result<_, E>`

When a method chain ending in `?` causes an E0277 because the
expression's `Result::Err` variant doesn't have a type that can be
converted to the `Result<_, E>` type parameter in the return type,
provide additional context of which parts of the chain can and can't
support the `?` operator.

```
error[E0277]: `?` couldn't convert the error to `String`
  --> $DIR/question-mark-result-err-mismatch.rs:28:25
   |
LL | fn bar() -> Result<(), String> {
   |             ------------------ expected `String` because of this
LL |     let x = foo();
   |             ----- this can be annotated with `?` because it has type `Result<String, String>`
LL |     let one = x
LL |         .map(|s| ())
   |          ----------- this can be annotated with `?` because it has type `Result<(), String>`
LL |         .map_err(|_| ())?;
   |          ---------------^ the trait `From<()>` is not implemented for `String`
   |          |
   |          this can't be annotated with `?` because it has type `Result<(), ()>`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following other types implement trait `From<T>`:
             <String as From<char>>
             <String as From<Box<str>>>
             <String as From<Cow<'a, str>>>
             <String as From<&str>>
             <String as From<&mut str>>
             <String as From<&String>>
   = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`
```

Fix #72124.

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