For the non-async case, early returns could escape the block so that the
error handling was skipped.

Annotating the return type of the closure improves the error message in
case of a mismatch between the function's return type and body.

Example code:
```
fn with_err() -> Result<(), String> {
    Ok(5)
}
```

Without return type annotation:
```
error[E0308]: mismatched types
 --> src/main.rs:2:37
  |
2 |   fn with_err() -> Result<(), String> {
  |  _____________________________________^
3 | |     Ok(5)
4 | | }
  | |_^ expected `()`, found integer
```

With return type annotation:
```
error[E0308]: mismatched types
 --> src/main.rs:3:8
  |
3 |     Ok(5)
  |        ^ expected `()`, found integer
```