Fix for init branch issue #306, 2010 client running against 2008 server

In the event of FeatureNotSupported exception, fall back to 2008 behavior.

I have tested this code with a 2010 client running against a 2008 server, as described in https://github.com/git-tfs/git-tfs/issues/306.  It works!  I couldn't figure out how to add any unit tests for this, though.  Maybe someone else knows better.