Handshake is now more resilient

Errors issued by the server for caching_sha2_password users after a fast
auth OK packet has been received (e.g. bad database errors) are now
reported correctly
Handshake now correctly detects protocol violation errors, like double
auth switches

close #469 
close #468

Author: anarthal

include/boost/mysql/client_errc.hpp

@@ -163,6 +163,12 @@ enum class client_errc : int
      * Use `async_start_execution` to start one.
      */
     not_engaged_in_multi_function,
+
+    /**
+     * \brief During handshake, the server sent a packet type that is not allowed in the current state
+     * (protocol violation).
+     */
+    bad_handshake_packet_type,
 };
 
 BOOST_MYSQL_DECL

include/boost/mysql/detail/make_string_view.hpp

@@ -100,6 +100,9 @@ inline const char* error_to_string(client_errc error)
     case client_errc::not_engaged_in_multi_function:
         return "The operation requires the connection to be engaged in a multi-function operation. "
                "Use async_start_execution to start one.";
+    case client_errc::bad_handshake_packet_type:
+        return "During handshake, the server sent a packet type that is not allowed in the current state "
+               "(protocol violation).";
     default: return "<unknown MySQL client error>";
     }
 }