This works by introspecting rustc's error output, using the JSON format
to determine whether it's a warning or error, then skipping it
altogether if it's a summary of the diagnostics printed.
Before:
```
src/main.rs:1:10: warning: trait objects without an explicit `dyn` are deprecated
src/main.rs:1:1: error[E0601]: `main` function not found in crate `wrong`
src/main.rs:1:9: error[E0038]: the trait `Clone` cannot be made into an object
error: aborting due to 2 previous errors; 1 warning emitted
error: could not compile `wrong`
```
After:
```
$ cargo check --message-format short
src/main.rs:1:10: warning: trait objects without an explicit `dyn` are deprecated
src/main.rs:1:1: error[E0601]: `main` function not found in crate `wrong`
src/main.rs:1:9: error[E0038]: the trait `Clone` cannot be made into an object
error: could not compile `wrong` due to 2 previous errors; 1 warning emitted
```