Increment metric when users attempt to use non-template repo w/ template setting enabled

[d12]

https://github.com/education/classroom/blob/d4246ec6ec1dca8399d6d965a7577ad430ad5374/app/models/concerns/starter_code_importable.rb#L29

Let's increment a ddog metric here, we can't tell if this is an error users are frequently seeing.

cc @andrewbredow @femmebot