Throw an error if plugin(s) / theme(s) aren't specified for update

`wp plugin update` and `wp theme update` both require either passed plugin(s) / theme(s), or an `--all` argument. Our synopsis parser isn't smart enough to require one argument or another, so this will be the catch-all instead.
Merge pull request #1144 from wp-cli/fix-update-error

`wp plugin update` runs with missing argument,