Fix parsing error message during handshake (Too many connections)

[clue]

The server may reject an incoming connection during authentication or even before that during the initial connection handshake. The initial handshake error message uses a slightly different message format and also semantically happens before executing a command, so we now make sure to properly reject the following authentication command.

Note:
In case the server sent a ERR packet as first packet it will happen before the client and server negotiated any capabilities. Therefore the ERR packet will not contain the SQL-state.

https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_connection_phase.html

Resolves / closes #81