Make ProtocolError inherit from CouchbaseNetworkError

Change-Id: I2162558e544fbc5e6746f1c239c51acf243287bd
Reviewed-on: http://review.couchbase.org/30801
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>