Pulp silently ignores redis connection issues

Version
3.45.0

Describe the bug
If you have issues with your redis cache config, Pulp will silently swallow those errors. Here are some scenarios:

1. If you set CACHE_ENABLED but forget to set REDIS_URL/REDIS_HOST
2. If your values for REDIS_HOST and/or REDIS_PORT are bad
3. If Pulp has network problems and can't reach redis

In all these scenarios, Pulp starts up no problem and doesn't report any issues. I would sort of expect Pulp to refuse to start if it encounters a problem connecting to redis but at the very least, I think it should loudly warn users that it can't connect to redis for whatever reason instead of silencing any connection issues.

Note: there was a somewhat related issue in which it was decided to have Pulp silently ignore redis errors but my interpretation is that it was only meant to silence issues after pulp had established a connection to redis. I'd probably argue that Pulp should at least log the errors though (as warnings).

Start by tracing the Redis cache configuration and startup connection handling described in the scenarios, then compare the behavior with issue 8997. Reproduce the missing REDIS_URL/REDIS_HOST, invalid host or port, and unreachable Redis cases; done means the chosen startup or warning behavior is covered by tests.