Prevent creating videoclips if ffmpeg is not installed on the server

Describe the bug

When ffmpeg is not installed you can launch videoclip creation but you should not be able to.

Steps to reproduce

1. Do not install ffmpeg
2. Create a videoclip

Expected behavior

We should get a warning and not be able to launch the videoclip creation.

Actual behavior

Videoclip starts (but of course never gets done).

Relevant logs and/or screenshots

ErrorException: unlink(/home/cloudpanel/htdocs/cpod.podcastindex.net/writable/temp/episode-71-specs-and-spacs-6853.309-6885.491BhEXuf-soundbite.mp3): No such file or directory in /home/cloudpanel/htdocs/cpod.podcastindex.net/app/Libraries/MediaClipper/VideoClipper.php:195
Stack trace:
#0 [internal function]: CodeIgniter\Debug\Exceptions->errorHandler()
#1 /home/cloudpanel/htdocs/cpod.podcastindex.net/app/Libraries/MediaClipper/VideoClipper.php(195): unlink()
#2 /home/cloudpanel/htdocs/cpod.podcastindex.net/app/Libraries/MediaClipper/VideoClipper.php(217): MediaClipper\VideoClipper->cleanTempFiles()
#3 /home/cloudpanel/htdocs/cpod.podcastindex.net/modules/Admin/Controllers/SchedulerController.php(64): MediaClipper\VideoClipper->generate()
#4 /home/cloudpanel/htdocs/cpod.podcastindex.net/vendor/codeigniter4/codeigniter4/system/CodeIgniter.php(825): Modules\Admin\Controllers\SchedulerController->generateVideoClips()
#5 /home/cloudpanel/htdocs/cpod.podcastindex.net/vendor/codeigniter4/codeigniter4/system/CodeIgniter.php(412): CodeIgniter\CodeIgniter->runController()
#6 /home/cloudpanel/htdocs/cpod.podcastindex.net/vendor/codeigniter4/codeigniter4/system/CodeIgniter.php(320): CodeIgniter\CodeIgniter->handleRequest()
#7 /home/cloudpanel/htdocs/cpod.podcastindex.net/public/index.php(41): CodeIgniter\CodeIgniter->run()
#8 {main}

Context

• Castopod: beta-5
• OS: Debian 10 + CloudPanel
• Browser: n/a
• Web server: Nginx
• ffmpeg not installed

Possible fixes