Adds connection error exception to handle redis connection failure

[akolson]

Summary

This pr adds a defensive check to ensure that Kolibri execution is halted in case it fails to connect to Redis.

References

Fixes #10551

Reviewer guidance

• Ensure that redis is not running on any of the ports specified below
• Run KOLIBRI_CACHE_BACKEND="redis" KOLIBRI_CACHE_LOCATION="redis:657" kolibri start on a pex file or local environment setup
• Kolibri should not start. The logs returned should be similar to the below;

INFO     2023-09-26 21:14:03,723 Option CACHE_BACKEND in section [Cache] being overridden by environment variable KOLIBRI_CACHE_BACKEND
INFO     2023-09-26 21:14:03,723 Option CACHE_LOCATION in section [Cache] being overridden by environment variable KOLIBRI_CACHE_LOCATION
INFO     2023-09-26 21:14:03,724 Option DEBUG in section [Server] being overridden by environment variable KOLIBRI_DEBUG
INFO     2023-09-26 21:14:03,724 Option DEBUG_LOG_DATABASE in section [Server] being overridden by environment variable KOLIBRI_DEBUG_LOG_DATABASE
INFO     2023-09-26 21:14:03,943 Running Kolibri with the following settings: kolibri.deployment.default.settings.base
WARNING  2023-09-26 21:14:03,986 Unable to connect to Redis: Error 8 connecting to redis:657. nodename nor servname provided, or not known.
Error: Traceback (most recent call last):
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/redis/connection.py", line 492, in connect
    sock = self._connect()
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/redis/connection.py", line 519, in _connect
    for res in socket.getaddrinfo(self.host, self.port, self.socket_type,
  File "/Users/akol/.pyenv/versions/3.8.13/lib/python3.8/socket.py", line 918, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno 8] nodename nor servname provided, or not known

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/core/apps.py", line 132, in check_redis_settings
    maxmemory_policy = helper.get_maxmemory_policy()
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/core/utils/cache.py", line 51, in get_maxmemory_policy
    return self.get("maxmemory-policy", default_value="noeviction")
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/core/utils/cache.py", line 34, in get
    return self.client.config_get(key).get(key, default_value)
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/redis/client.py", line 902, in config_get
    return self.execute_command('CONFIG GET', pattern)
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/redis/client.py", line 772, in execute_command
    connection = pool.get_connection(command_name, **options)
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/redis/connection.py", line 1142, in get_connection
    connection.connect()
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/redis/connection.py", line 497, in connect
    raise ConnectionError(self._error_message(e))
redis.exceptions.ConnectionError: Error 8 connecting to redis:657. nodename nor servname provided, or not known.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/utils/cli.py", line 193, in invoke
    initialize(**get_initialize_params())
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/utils/main.py", line 295, in initialize
    _setup_django()
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/utils/main.py", line 153, in _setup_django
    django.setup()
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/django/__init__.py", line 27, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/Users/akol/.pyenv/versions/3.8.13/envs/kolibri-py3.8.13/lib/python3.8/site-packages/django/apps/registry.py", line 116, in populate
    app_config.ready()
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/core/apps.py", line 45, in ready
    self.check_redis_settings()
  File "/Users/akol/Desktop/leDev/kolibri/kolibri/core/apps.py", line 172, in check_redis_settings
    raise RedisConnectionError("Unable to connect to Redis: {}".format(str(e)))
kolibri.core.errors.RedisConnectionError: Unable to connect to Redis: Error 8 connecting to redis:657. nodename nor servname provided, or not known.

• Look out for the Unable to connect to Redis... in the list of logs(RedisConnectionError should be raised)
• Also, running KOLIBRI_CACHE_BACKEND="redis" KOLIBRI_CACHE_LOCATION="redis:657" kolibri services should yield the same results

---

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b5.dev0_git.321.g09968f26.pex
Windows Installer (EXE)	kolibri-0.16.0b5.dev0+git.321.g09968f26-unsigned.exe
Debian Package	kolibri_0.16.0b5.dev0+git.321.g09968f26-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b5.dev0+git.321.g09968f26-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b5.dev0+git.321.g09968f26-0.1.0-debug.apk
TAR file	kolibri-0.16.0b5.dev0+git.321.g09968f26.tar.gz
WHL file	kolibri-0.16.0b5.dev0+git.321.g09968f26-py2.py3-none-any.whl

[rtibbles]

Behaviourally checks out - one non-blocking suggestion. Interested in @bjester's review in case he had anything additional in mind when filing the original issue.

[rtibbles]

All comments addressed, and this seems to do what is needed.