Allow a context to be passed in for error handling

As pointed out by Tom Lane, we can allow other users of the error
handler callbacks to provide their own memory context by adding
the context to use to ErrorData and using that instead of explicitly
using ErrorContext.

This then allows GetErrorContextStack() to be called from inside
exception handlers, so modify plpgsql to take advantage of that and
add an associated regression test for it.

src/test/regress/sql/plpgsql.sql

@@ -3927,3 +3927,59 @@ select outer_outer_func(20);
 drop function outer_outer_func(int);
 drop function outer_func(int);
 drop function inner_func(int);
+
+-- access to call stack from exception
+create function inner_func(int)
+returns int as $$
+declare
+  _context text;
+  sx int := 5;
+begin
+  begin
+    perform sx / 0;
+  exception
+    when division_by_zero then
+      get diagnostics _context = pg_context;
+      raise notice '***%***', _context;
+  end;
+
+  -- lets do it again, just for fun..
+  get diagnostics _context = pg_context;
+  raise notice '***%***', _context;
+  raise notice 'lets make sure we didnt break anything';
+  return 2 * $1;
+end;
+$$ language plpgsql;
+
+create or replace function outer_func(int)
+returns int as $$
+declare
+  myresult int;
+begin
+  raise notice 'calling down into inner_func()';
+  myresult := inner_func($1);
+  raise notice 'inner_func() done';
+  return myresult;
+end;
+$$ language plpgsql;
+
+create or replace function outer_outer_func(int)
+returns int as $$
+declare
+  myresult int;
+begin
+  raise notice 'calling down into outer_func()';
+  myresult := outer_func($1);
+  raise notice 'outer_func() done';
+  return myresult;
+end;
+$$ language plpgsql;
+
+select outer_outer_func(10);
+-- repeated call should to work
+select outer_outer_func(20);
+
+drop function outer_outer_func(int);
+drop function outer_func(int);
+drop function inner_func(int);
+