Makes bin/flapjack-populator explicitly whinge about `-f` instead of dying.

Presently:

```
FLAPJACK_ENV=development bundle exec bin/flapjack-populator import-contacts tmp/dummy_entities.json -c etc/flapjack_config.yaml 
bin/flapjack-populator:83:in `load': strict_parse() expected a String or IO Object. (ArgumentError)
        from bin/flapjack-populator:83:in `<main>'
```

This PR:

```
No import file provided with --from, eg. --from contacts.json

Usage: flapjack-populator COMMAND [OPTIONS]

Commands:
     import-entities
     import-contacts
     purge-events    purge queued monitoring events

Options
    -c, --config [PATH]              PATH to the config file to use
    -f, --from [FILE]                path to the FILE to import
```

(Just a minor thing I banged into while setting up a dev environment for #391.)