contact sheet: error if the theme file could not be loaded

Fixes: #18

extensions/contact_sheet/dlg-contact-sheet.c

@@ -354,6 +354,7 @@ add_themes_from_dir (DialogData *data,
 			g_free (buffer);
 			g_object_unref (file);
 			g_object_unref (file_info);
+			continue;
 		}
 
 		theme = gth_contact_sheet_theme_new_from_key_file (key_file);