Throw an exception if a package is created without setting the author's email

`artisan workbench {namespace/package}` will create the files for a package even if you didn't set up the configuration for workbench.

When the command try to `composer install`, composer will throw `Composer\Json\JsonValidationException`.

The context isn't very clear since one cannot find the exception anywhere in the code, so this should clarify the error.