More precise error messages for uninitialized fields in the presence of inheritance

tests/notnil/tnotnil_in_objconstr.nim

@@ -1,13 +1,17 @@
 discard """
-  errormsg: "The Foo type requires the following fields to be initialized: bar"
-  line: "13"
+  errormsg: "The Foo type requires the following fields to be initialized: bar, baz"
+  line: "17"
 """
 {.experimental: "notnil".}
 # bug #2355
 type
-  Foo = object
+  Base = object of RootObj
+    baz: ref int not nil
+
+  Foo = object of Base
     foo: ref int
     bar: ref int not nil
+
 var x: ref int = new(int)
 # Create instance without initializing the `bar` field
 var f = Foo(foo: x)