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

[alexcrichton]

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.