MXS-2603: Treat WSREP errors as ignorable errors

When a query returns a WSREP error, most of the time it is not something
the client application is expecting. To prevent this from affecting the
client, it can be treated the same way a transaction rollback is treated:
ignore the error and try again.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -229,6 +229,7 @@ private:
     bool lock_to_master();
     bool is_locked_to_master() const;
     bool supports_hint(HINT_TYPE hint_type) const;
+    bool handle_ignorable_error(mxs::RWBackend* backend);
 
     inline bool can_retry_query() const
     {