Only update call center owner on creation

Don't merge unless you are @snopoke 
This is hard for me to parse. `None` -> no error ... `if result[1]` -> notify. Would it be clearer to change the comment to 
```
# Second item contains error message; None if no error
```
What do you think of this instead (assumed `result` is a two-item sequence, may require tweaking)?
```py
for result, error in results:
    if error:
        ... (commcare_user.username, str(error))
```