agreed, it is too terse. This looks like a basic development msg that was intended to be fleshed out later but got over-looked.
The problem was that I did not realise I was, as far as kicad was concerned, effectively creating another separate circuit and placing it on top of the existing one by trying to attach new track to a pad of the imported net.
Similarly on my final problem with the gnd tracks, I was trying to make legit connection but the existing name was blocking since the track had a net name that was not GND. This is a very different scenario which produces exactly the same kind of messages. The information in the error msg should be explicit enough to enable the user to distinguish these two situations.
My other issue with all this is that it was far too discrete. As I noted initially this is a show stopper and it needs to be flagged with a model dialogue error msg, not something as far away as possilbe from the screen centre where the user’s focus is and which disappears as soon as he moves the mouse or clicks something.