[g-ir-compiler] Slightly less lame error messages

To make things really better we should track the line origin of
element from the .gir file (and actually we need to do better
checking in the scanner), but this is slightly less lame.

girnode.c

@@ -1090,7 +1090,7 @@ find_entry_node (GIrModule   *module,
       goto out;
     }
 
-  g_warning ("Entry '%s' not found", name);
+  g_ir_module_fatal (module, 0, "Type reference '%s' not found", name);
 
  out: