Minor changes to error handling and VolumeDataAccessManager interface reference counting

• Remove special handling of error code -4 which is not needed anymore.
• Make the GetVolumeDataAccessManager function add the reference to the interface instead of doing it in the VolumeDataAccessManager constructor in order to align with commerical VDS. The main argument for doing it this way is that the operation can be 100% thread safe when you make sure to add the reference while you are holding a mutex preventing the interface from being destroyed.