[6.x] Update error message if redis extension is not loaded

[ericdowell]

Based on #29864 I would say it is time for better error messages than what has always been provided (pre-6.x).

After spending around 30 minutes to an hour figuring out that the PHP Redis extension was installed but not enabled I found myself displeased with the unhelpful default error message. To help all future developers we should be detecting if redis extension is not loaded and then provide a more helpful message.

Given the class PhpRedisConnector is only used for the phpredis driver, no checks other than !extension_loaded('redis') is needed.

Tested on machine without extension loaded:

forge@solitary-thunder:~$ php -a
Interactive mode enabled

php > var_dump(extension_loaded('redis'));
bool(false)
php >

and one with the extension loaded:

forge@fathomless-breeze:~$ php -a
Interactive mode enabled

php > var_dump(extension_loaded('redis'));
bool(true)
php >