PG-186: Add support to monitor query execution plan.

This requires refactoring of code to add this functionality. Along with
that this patch contains regression test cases.

regression/sql/error.sql

@@ -0,0 +1,14 @@
+CREATE EXTENSION pg_stat_monitor;
+SELECT pg_stat_monitor_reset();
+SELECT 1/0;   -- divide by zero
+SELECT * FROM unknown; -- unknown table
+ELECET * FROM unknown; -- syntax error
+
+do $$
+BEGIN
+RAISE WARNING 'warning message';
+END $$;
+
+SELECT query, elevel,sqlcode, message FROM pg_stat_monitor ORDER BY query COLLATE "C";
+SELECT pg_stat_monitor_reset();
+DROP EXTENSION pg_stat_monitor;