Might be better for Graph.topsort/1 to return :error and {:ok, sorted_graph}?

Dialyzer complains `The test [any()] == 'false' can never evaluate to 'true'` when I try to check the output of `Graph.topsort/1`. Apparently the typespec annotation confused it. I guess one could either change the return value to `:error | {:ok, sorted_graph}` or change the typespec so that Dialyzer doesn't find it problematic? Or am I doing something wrong here.