return error when instructor names an assignment using an existing name

[kjdav]

OST doesn't allow an instructor to create assignments with duplicate names. When a user tries to do this, OST simply does not create the assignment but it's unclear why. We should return an error so it's clear what the problem is and how to resolve it.

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