Scenario should have at least one alternative selected

[jkiviluo]

When executing a workflow with scenario filters, it should be checked that each scenario includes at least one alternative. Otherwise there is no data and tools can have uninformative error messages (the first thing that is missing instead of pointing to the lack of alternative).

This could also be highlighted in DB editor - each scenario name in scenario list marked e.g. red, if it has no alternative. However, this is not sufficient, since execution can happen without DB editor.