backend: refine the error message when require TLS

[djshow832]

What problem does this PR solve?

Issue Number: close #312

Problem Summary:
When the TiDB disables TLS and TiProxy enables require-backend-tls, the client receives error (1105, 'Verify TiDB capability failed, please upgrade TiDB'). This makes users confused when users deploy the TiDB cluster by themselves. We should tell them that TiDB should enable TLS.

What is changed and how it works:

• Change error message when either TiDB or TiProxy doesn't enable TLS

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

1. set require-backend-tls = true
2. Disable TiDB TLS
3. Connect with client and see error: ERROR 1105 (HY000): Require TLS config on TiDB when require-backend-tls=true. In the log: [new connection failed] [connID=0] [client_addr=127.0.0.1:53490] [proxy-protocol=false] [backend_addr=127.0.0.1:4000] ["quit source"="proxy error"] [error="backend doesn't enable TLS"]
4. Enable TiDB TLS and disable TiProxy TLS by setting security.sql-tls.skip-ca = false.
5. Connect with client and see error: ERROR 1105 (HY000): Require TLS config on TiProxy when require-backend-tls=true. In the log: [new connection failed] [connID=0] [client_addr=127.0.0.1:55840] [proxy-protocol=false] [backend_addr=127.0.0.1:4000] ["quit source"="proxy error"] [error="tiproxy doesn't enable TLS"]

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-05 02:12:29.036265867 +0000 UTC m=+2411513.585281838: ☑️ agreed by xhebox.