Usage error should show there is a --help option

For instance

```
$ ./scancode 
Usage: scancode [OPTIONS] <input> <output_file>

Error: Missing argument "input".None
```

It would be better if we had something such as:

```
$ ./scancode 
Usage: scancode [OPTIONS] <input> <output_file>
Try 'scancode --help' for help on options and arguments.

Error: Missing argument "input".None
```
