To me, "Errands" is not a project, but a context. Projects are independent of contexts. As an example, I might have "Buy brick anchors at the hardware store" as part of my project of "Renovate the living room", but it would fall into context "Errands".
I view context as being a "While I'm here" kind of system. So if I'm grocery shopping, I check my "errands" context to see if there's anything I can do that's on the way to the store.
With this setup, items in context "Errands" could fall under a project OR be a single action.
I don't think Omnifocus does require actions to be in a project! I just have two top folders called "Work" and "Personal", and under each of those folders I have a sub-folder called "Single Actions" where I store all my one-offs. But each of those one-off actions does have a context.