Check if the API is in error state then abort

Property definitions

github $ Loader :: check_error
	# Check if the API is in error state then abort
	fun check_error(api: GithubAPI, message: nullable String) do
		var err = api.last_error
		if err != null then
			error message or else err.message
		end
	end
lib/github/loader.nit:408,2--414,4