fix error block location in radio buttons

Currently, radio widget error messages appear between the final checkbox and the final label.  Instead, use CheckboxesWidget's invisible input method to display errors on radio widgets.

gluon/sqlhtml.py

@@ -461,7 +461,12 @@ class RadioWidget(OptionsWidget):
             opts.append(child(tds))
 
         if opts:
-            opts[-1][0][0]['hideerror'] = False
+            opts.append(
+                INPUT(requires=attr.get('requires', None),
+                      _style="display:none;",
+                      _disabled="disabled",
+                      _name=field.name,
+                      hideerror=False))
         return parent(*opts, **attr)