Converts a SignedBinaryID to the equivalent AFF4 URN.

def _SignedBinaryURNFromID(binary_id
                          ):
  """Converts a SignedBinaryID to the equivalent AFF4 URN."""
  binary_type = binary_id.binary_type
  if binary_type == rdf_objects.SignedBinaryID.BinaryType.PYTHON_HACK:
    return GetAFF4PythonHackRoot().Add(binary_id.path)
  elif binary_type == rdf_objects.SignedBinaryID.BinaryType.EXECUTABLE:
    return GetAFF4ExecutablesRoot().Add(binary_id.path)
  else:
    raise ValueError("Unknown binary type %s." % binary_type)