Surface server errors during SSL negotiation

[aryansk]

Summary

Preserve the PostgreSQL server's diagnostic when it returns an early error while asyncpg is waiting for the one-byte SSL negotiation response.

Changes

• recognize pre-auth responses beginning with the PostgreSQL ErrorResponse marker (E)
• decode and surface the server-provided text as an asyncpg InterfaceError
• retain the existing S, advisory N, and generic fallback behavior
• add a unit regression test using the reported pre-auth response shape

Test plan

python -m unittest tests.test_connect.TestTLSUpgradeProto.test_error_response_preserves_server_message

Fixes #1317