Fix bug in batch ID field renderer, when displayed for new batch

Hopefully this is a good idea..

tailbone/forms/renderers/batch.py

@@ -40,9 +40,15 @@ class BatchIDFieldRenderer(fa.FieldRenderer):
     Renderer for batch ID fields.
     """
     def render_readonly(self, **kwargs):
-        batch_id = self.raw_value
-        if batch_id:
-            return '{:08d}'.format(batch_id)
+        try:
+            batch_id = self.raw_value
+        except AttributeError:
+            # this can happen when creating a new batch, b/c the default value
+            # comes from a sequence
+            pass
+        else:
+            if batch_id:
+                return '{:08d}'.format(batch_id)
         return ''