Allow customisation of Redis session handler errors

The changes are covered by unit tests.

Setup

1. Install the PHP Redis extension (replace "X" as your server's PHP version, e.g. 8.0)

   sudo apt update && sudo apt install -y phpX-redis

2. Restart your web server.
3. Confirm the redis PHP extension is enabled: "php -i | grep redis" - You should see a bunch of lines related to redis.
4. Install redis. e.g. using Docker

   docker run --name redis -p 6379:6379 -d redis

5. Setup redis as the session handler, e.g. copy the following to your config.php:

   $CFG->session_handler_class = '\core\session\redis';

   $CFG->session_redis_host = '127.0.0.1';

   $CFG->session_redis_port = 6379;

   $CFG->session_redis_acquire_lock_timeout = 2;

6. Install the session breaker plugin to help with testing:

   git clone https://github.com/catalyst/moodle-tool_sessionbreaker/ admin/tool/sessionbreaker

Test

1. Visit this page, which will lock the session for 20 seconds: /admin/tool/sessionbreaker/bad.php?wait=20
2. In a second tab immediately load this page: /admin/tool/sessionbreaker/good.php
3. Confirm after roughly 2 seconds it throws an exception eg

   Unable to obtain session lock for sid: pvga0j242m within 0 days 0 hours 0 minutes 2 seconds.

   It is likely that another page ([pid 1758327] workstation:/admin/tool/sessionbreaker/bad.php?wait=20), possibly in another browser tab, has caused this due to any of the following reasons:

   it has a long session lock; or

   the session lock was never released due to an error.

   You can try to close and re-open your browser or wait until the session lock timeout (0 days 20 hours 0 minutes 0 seconds) has passed. If this error persists, please notify the server administrator.