SSLMode=prefer terminates connection with error instead of unsecure connection

[SovakPaleny]

Summary

Our applications always use SSLMode=prefer to connect to databases, including unsecured ones. It works well, but not with secretless-broker. SSLMode=prefer should be a safe option for capability autodetection.

Also, SSLMode=require should send the correct message to the client instead of a generic error.

Steps to Reproduce

$ PGSSLMODE=prefer psql -h localhost -p 55432 -U postgres postgres
psql: could not connect to server: Connection refused
        Is the server running on host "localhost" (::1) and accepting
        TCP/IP connections on port 55432?
FATAL:  SSL not supported

$ PGSSLMODE=require psql -h localhost -p 55432 -U postgres postgres
psql: could not connect to server: Connection refused
        Is the server running on host "localhost" (::1) and accepting
        TCP/IP connections on port 55432?
FATAL:  SSL not supported

Expected Results

Secretless broker should correctly signalize unsupported SSL to the client so the client can continue with an unsecured connection if required.

$ PGSSLMODE=prefer psql -h localhost -p 5432 -U postgres postgres
psql (10.15, server 13.1 (Debian 13.1-1.pgdg100+1))
WARNING: psql major version 10, server major version 13.
         Some psql features might not work.
Type "help" for help.

postgres=# 

Correct signaling with SSLMode=require

$ PGSSLMODE=require psql -h localhost -p 5432 -U postgres postgres
server does not support SSL, but SSL was required

Actual Results (including error logs, if applicable)

2021/02/05 11:02:37 [DEBUG] pg: New connection on 127.0.0.1:55432.
2021/02/05 11:02:37 [DEBUG] pg: Handling connection 127.0.0.1:59066 -> 127.0.0.1:55432
2021/02/05 11:02:37 [DEBUG] pg: s.Client version: 80877103, (SSL mode: true)
2021/02/05 11:02:37 [ERROR] pg: Failed on handle connection: failed on connect: SSL not supported

Additional Information

This issue blocks us from the next steps in the evaluation process so I prepared a patch that fixes the issue so we can continue.