Tweak format string error to point at arguments always

Add secondary span labels with no text to make it clear when there's a
mismatch bewteen the positional arguments in a format string and the
arguments to the macro. This shouldn't affect experienced users, but it
should make it easier for newcomers to more clearly understand how
`format!()` and `println!()` are supposed to be used.

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^   -
```

instead of

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^
```

0 files changed, 0 insertions, 0 deletions