Handle bodyless NMVB (w/CCCP) as a 'soft' error rather than a 'hard' error

Failed not-my-vbucket parsing should be treated as a 'soft' error (which triggers a throttled refresh) rather than a hard error (which triggers a non-throttled refresh).

I've never seen a vbconfig parsing error, and in the even when such occurs, an application can wait 10 seconds (or adjust the throttle). on the other hand, we've seen CPU usage increases (along with additional config requests) on nmv responses without a vbucket config (when using cccp).