[MERGE][0.16] Allow new smart client to work with old (<= 0.15) smart servers.

[Martin Pool]

Martin Pool has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
I agree with Robert.  Also it's a bit obscure why you're raising an 
exception there, so please just add a comment like "decline to open it 
and the vfs-based transport will do it."

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20070424114927.GC17953%40steerpike.home.puzzling.org%3E