obfs4proxy should be more helpful if state file is empty
We had a user on IRC who ran into the following error message:
[warn] Server managed proxy encountered a method error. (obfs4 failed to load statefile '/var/db/tor/pt_state/obfs4_state.json': unexpected end of JSON input)
It turns out that the user's state file was empty. Removing the state file and then having obfs4proxy re-create it fixed the problem. Obfs4proxy should realise that the state file is empty (was opposed to corrupt) and either re-create it itself or advise the user to delete it and try again.