backend, net: add more error sources

[djshow832]

What problem does this PR solve?

Issue Number: close #404

Problem Summary:
The error sources are not enough to judge whether the client quits normally or abnormally.

What is changed and how it works:

• Add more kinds of error sources.
• It was unreadable to set the error source for OnHandshake and reset it, so I added an ErrorSource parameter for OnHandshake& OnConnClose and removed the interface ConnContext.ErrorSource().
• Add more predefined errors and wrap all the errors with these predefined errors so that we can track them correctly.
• Add Error2Source to convert an error to an ErrorSource.
• Remove UserError. Convert tracked errors to MySQL errors by ErrToClient().

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
• 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:

• 2023-12-01 07:30:39.233183769 +0000 UTC m=+1167067.898409963: ☑️ agreed by xhebox.