Warn user when attempting to create or rename a collection to a name that already exists.

In some cases syncing collections will attempt to create collections with the same name locally. This can happen if the user has used a reserved name for their set at the site (like "Library" or "Public Profile"). Or in some cases it is possible to end up with different sets at 500px to have the same name (see https://github.com/500px/api-documentation/issues/88).

This change adds error messages explaining to the user why the collection wasn't created or renamed locally.