Don't allow user to Continue from connecting way of a closed loop

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There are a bunch of changes forthcoming in #3676 which should prevent duplicate nodes along ways from happening.

A consequence of that is now way.addNode will try to preserve circularity (keep closed ways closed), so continuing a way from the connecting node of a closed loop way could cause the continued segment to loop back over itself.

And anyway it's confusing why one vertex allows it and none others along the way do.

If a user really wants to extend the way from this point, they can just draw another way and connect them there.

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This was done in cb35873