Print an error when an output format is requested without `--raw` on `benchmark`

I tried to collect some benchmarks today and passed `-o foo.json -f json` and was perplexed as the output wasn't in JSON form. I later learned, however, that I should pass `--raw` which then output JSON. I think it would have helped myself if the usage of `-f` or `--output-format` was an error without the `--raw` flag in this case.