Catch unhandled exceptions in health checks

What does this MR do?

Fixes #23927 (closed)

Right now when redis is down, the redis readiness check raises Redis::CannotConnectError (Error connecting to Redis on localhost:6380 (Errno::ECONNREFUSED))

This unhandled exception then makes the readiness endpoint return a static HTML error page when run in production mode.

Instead, we should catch unhandled exceptions at the check level and return a failed check result.

With this MR, the endpoint now returns the expected result:

{
   "db_check":{
      "status":"ok"
   },
   "redis_check":{
      "status":"failed",
      "message":"unexpected Redis check result: Error connecting to Redis on localhost:6380 (Errno::ECONNREFUSED)"
   },
   "cache_check":{
      "status":"failed",
      "message":"unexpected Cache check result: Error connecting to Redis on localhost:6380 (Errno::ECONNREFUSED)"
   },
   "queues_check":{
      "status":"failed",
      "message":"unexpected Queues check result: Error connecting to Redis on localhost:6381 (Errno::ECONNREFUSED)"
   },
   "shared_state_check":{
      "status":"failed",
      "message":"unexpected Shared_state check result: Error connecting to Redis on localhost:6382 (Errno::ECONNREFUSED)"
   },
   "gitaly_check":{
      "status":"ok",
      "labels":{
         "shard":"default"
      }
   }
}

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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