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.

include/maxscale/protocol/rwbackend.hh

@@ -88,6 +88,13 @@ public:
             return rv;
         }
 
+        bool is_wsrep_error() const
+        {
+            return m_code == 1047
+                   && m_sql_state == "08S01"
+                   && m_message == "WSREP has not yet prepared node for application use";
+        }
+
         bool is_unexpected_error() const
         {
             switch (m_code)