Say "ES is not ready" when ES readiness probe fails

[kunisen]

Description

ECK is using ES readiness TCP port for readiness check now:
https://www.elastic.co/guide/en/elasticsearch/reference/current/advanced-configuration.html#readiness-tcp-port

However, when ES is not ready, it says Connection refused which is a bit misleading.

nc: connect to 127.0.0.1 port 8080 (tcp) failed: Connection refused

For example, as mentioned in #8042 (comment), disabling xpack.security.http.ssl.enabled causes ES node not deemed to be ready, but it still reports Connection refused error, which is logically hard to correlate the 2 things.

It would be great if we can catch this error and say it in a more user-friendly way, for example,
"ES node is not ready. A node is deemed ready when it has successfully joined a cluster. In a single node configuration, the node is said to be ready, when it’s able to accept requests. Reference: https://www.elastic.co/guide/en/elasticsearch/reference/current/advanced-configuration.html#readiness-tcp-port."

Motivation is to improve the readability of the error message and reduce the potential confusion.

[pebrc]

I think this is a good idea. Given that this is a shell script we can hide away the error and output a custom message.