Fix confusing error message for sub_instant

When subtracting an Instant from another, the function will panick when `RHS > self`, but the error message confusingly displays a different error:

```rust
let i = Instant::now();
let other = Instant::now();
if other > i {
    println!("{:?}", i - other);
}
```
This results in a panic:
```
thread 'test_instant' panicked at 'other was less than the current instant', libstd/sys/unix/time.rs:292:17
```

0 files changed, 0 insertions, 0 deletions