Right now if you connect a geometry input in the modifier stack you get a warning in the UI:
If I connect a vector3 (e.g., Object Info -> Location) in the group output it throws an error regarding "The node group must have a single geometry output".
I don't think we should throw any error re: output nodes, unless there is NONE Geometry output. Since users can re-use the nodes as either groups or modifiers I don't think they need to be warned about the output.
Suggestion of error when there is no "Geometry Output" socket: "The node group must one geometry output".