E0004 bonus update: spans can't cross nodes

From https://github.com/rust-lang/rust/pull/35485

After talking with @arielb1 (and seeing @nrc's comments) - let's actually change the span to be one we can do safely:

From:

```
match x {
^^^^^^^ pattern `HastaLaVistaBaby` not covered
```

To:

```
match x {
      ^ pattern `HastaLaVistaBaby` not covered
```

CC: @KiChjang
