backend, net: return handshake errors to the client

[djshow832]

What problem does this PR solve?

Issue Number: close #188

Problem Summary:
Some errors are not returned to the client, compared with TiDB.

What is changed and how it works:

• Move UserError from package backend to net because net uses it.
• Return ErrNotSupportedAuthMode to the client when frontend capability mismatches. This is consistent with TiDB.
• Return ErrMalformPacket when packet size is less than 32. This is consistent with TiDB.
• Return error to the client when TLS handshake fails. This is consistent with TiDB.
• Embed logger into PacketIO.

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

[djshow832]

This is already checked in ReadSSLRequestOrHandshakeResp.