- Adds a new `ProjectMovedError` class to encapsulate that error
  condition. Inherits from `NotFoundError` so existing rescues should
  continue to work.
- Separating that condition out of `NotFoundError` allowed us to
  simplify the `raise_not_found` helper and avoid repeating the literal
  string.
- Spec makes use of `ERROR_MESSAGES` hash to avoid repeating literal
  error message strings.