I've written a workflow level ErrorMessage as an attached property on the root workflow activity, at design time. I made the attached property for ErrorMessage written as an InArgument (to support some custom control that works well with InArguments).
Here's some of the design time code:
AttachedProperty<InArgument<string>> ErrorMessage = new AttachedProperty<InArgument<string>>
OwnerType = typeof(ActivityBuilder),
IsBrowsable = true,
Name = "ErrorMessage",
Getter = (mi =>
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