Considering that this block is only entered if there are no errors, we can just do

```py
try:
    word_inst.add_child(new_word, author)
except DuplicateWordException:
    errors.append('Duplicate word detected.')
else:
    story_inst.prune()
    response.redirect("/story/{}".format(story_inst.story_id))
    return
```
as the else block of a try/except is only entered if no exceptions are throw in the try block, and the only place errors are going to be added is in the except block, where an exception has to have occured