Return error 1290

[alpes214]

Description:
Fix for issue #2226. Return error code 1290 to client when backend server started with --read-only
Testing:
Before fix when client inserts record to the table proxysql disconnects it and prints warning
Server:

2019-12-17 09:13:58 MySQL_Session.cpp:4272:handler(): [WARNING] Error during query on (0,127.0.0.1,3306): 1290, The MySQL server is running with the --super-read-only option so it cannot execute this statement

CLI:

mysql> insert into t (a) values ('1');
ERROR 2013 (HY000): Lost connection to MySQL server during query

After fix client is not disconnected. Proxysql returns error 1290 and error message to the client. Proxysql also prints same warning into the log.

mysql> insert into t (a) values ('1');
ERROR 1290 (HY000): The MySQL server is running with the --super-read-only option so it cannot execute this statement