*: handle more nil error cases

[xhebox]

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: close None

Problem Summary: There are two problems.

1. p.wrap is an error intended to wrap other errors, it should be appended to the error list of pnet.Conn.Close().
2. Change how errors.Wrap handles nil.

This fixes [1.667360865291243e+09] [error] [main.proxy] [close connection fails] [connID=0] [remoteAddr="[::1]:51379"] [error="failed to close client connection:\n\tfailed to close the connection:\n\tthis is an error from client"].

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

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None