[5.5] Add better error message for missing app key

[thecrypticace]

Running php artisan key:generate is done automatically by laravel new project_name but the experience is still subpar when cloning an existing repo.

This is especially an issue on teams when cloning existing repos. Even more so with new team members who may not know Laravel (or PHP) that well.

The current error message "The only supported ciphers are AES-128-CBC and AES-256-CBC with the correct key lengths." is not helpful in determining what to do. It tells you that something is wrong but doesn't pinpoint what that thing is.

Because the most common occurrence of this exception is an empty or missing APP_KEY I thought it would be beneficial to check for this and present a more helpful exception.

[vlakoff]

You should move your added code above the startsWith test.

[thecrypticace]

Well, this has the advantage of catching APP_KEY=base64: but I guess that's not very likely.