Should compute invalid_sizes to use in error. Also, probably clearer to do as two lines
```
invalid_sizes = app_gear_sizes - valid_sizes
return render_error(:unprocessable_entity, "App gear sizes #{invalid_sizes.to_sentence} are not valid for this domain. Allowed sizes: #{valid_sizes.to_sentence}.", 134, "gear_size") if (invalid_sizes.length > 1)
return render_error(:unprocessable_entity, "App gear size #{invalid_sizes.to_sentence} is not valid for this domain. Allowed sizes: #{valid_sizes.to_sentence}.", 134, "gear_size") if (invalid_sizes.length == 1)
```