add the error number for unhandled user-defined exceptions (#192)

Author: petermueller

lib/myxql/protocol/server_error_codes.ex

@@ -27,6 +27,7 @@ defmodule MyXQL.Protocol.ServerErrorCodes do
     {1451, :ER_ROW_IS_REFERENCED_2},
     {1452, :ER_NO_REFERENCED_ROW_2},
     {1461, :ER_MAX_PREPARED_STMT_COUNT_REACHED},
+    {1644, :ER_SIGNAL_EXCEPTION},
     {1792, :ER_CANT_EXECUTE_IN_READ_ONLY_TRANSACTION},
     {1836, :ER_READ_ONLY_MODE},
     {3819, :ER_CHECK_CONSTRAINT_VIOLATED}