erp5_post: limit follow_up results

- fails if 0
- fails if too much
- also applied for predecessor

bt5/erp5_post/SkinTemplateItem/portal_skins/erp5_post/PostModule_createHTMLPost.py

@@ -4,12 +4,8 @@ portal = context.getPortalObject()
 post_module = portal.post_module
 
 # get the related object
-object_list = portal.portal_catalog(relative_url=follow_up) # with id keyword, this function will return a sequence data type which contains one element.
-follow_up_object = None
-if object_list:
-  follow_up_object = object_list[0].getObject()
-else:
-  raise LookupError("Target follow up object not found")
+follow_up_object, = portal.portal_catalog(relative_url=follow_up, limit=2)
+follow_up_object = follow_up_object.getObject()
 
 now = DateTime()
 post_edit_kw = {
@@ -18,7 +14,7 @@ post_edit_kw = {
   "text_content": data,
 }
 if predecessor is not None:
-  predecessor_value, = portal.portal_catalog(relative_url=predecessor)
+  predecessor_value, = portal.portal_catalog(relative_url=predecessor, limit=2)
   post_edit_kw["predecessor_value"] = predecessor_value.getObject()
 post = post_module.newContent(
   portal_type='HTML Post',