Update formatting ci to use --error-on-unformatted

This makes ci fail if rustfmt bails because it's unable to format the
code.

See <rust-lang/rustfmt#1976 (comment)>
for discussion of this option.

Author: dzfranklin

.github/workflows/ci.yml

@@ -135,4 +135,4 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: fmt
-          args: --all -- --check
+          args: --all -- --check --unstable-features --error-on-unformatted