Describe the bug

PHPCS throws a fatal error when the user passes an invalid generator.

To reproduce

Steps to reproduce the behavior:

1. Run phpcs test.php --generator=InvalidGenerator --standard=Generic
2. See the error message displayed

$ phpcs test.php --generator=InvalidGenerator --standard=Generic PHP Fatal error: Uncaught Error: Class "PHP_CodeSniffer\Generators\InvalidGenerator" not found in src/Runner.php:98 Stack trace: #0 bin/phpcs(14): PHP_CodeSniffer\Runner->runPHPCS() #1 {main} thrown in src/Runner.php on line 98

Expected behavior

PHPCS should display a user-friendly error message informing the user that the generator does not exist and potentially listing the available generators.

Please confirm

• I have searched the issue list and am not opening a duplicate issue.
• I have read the Contribution Guidelines and this is not a support question.
• I confirm that this bug is a bug in PHP_CodeSniffer and not in one of the external standards.
• I have verified the issue still exists in the master branch of PHP_CodeSniffer.