return error when instructor names a topic using an existing name

[kjdav]

OST doesn't allow an instructor to create topics with duplicate names. However when a user tries to name a topic using an existing name, OST does not return an error. Instead, it defaults to the topic's "default" name (e.g., Topic 12). We should return an error so it's unclear what the problem is and how to resolve it.

To do: when an instructor names a topic using a name that already exists, return an error message that says "A topic with this name already exists. Please rename this topic."

Related to issue #338.