poor "readonly transport" message when trying to write to an import branch

[Martin Pool]

If you try to write to an import branch, even one you own where imports are suspended, you get "lockdir failed... readonly transport" like in <http://stackoverflow.com/questions/2487545/how-to-get-local-bzr-commits-to-server/2489092>. It would be nice to give a clearer message about why.