Improve basic support for unit/pack info when viewing product details

tailbone/forms/core.py

@@ -728,6 +728,8 @@ class Form(object):
         return True
 
     def render_field_readonly(self, field_name, **kwargs):
+        if field_name not in self.fields:
+            return ''
         label = HTML.tag('label', self.get_label(field_name), for_=field_name)
         field = self.render_field_value(field_name) or ''
         field_div = HTML.tag('div', class_='field', c=[field])