Checks that a time-range has both start and end timestamps set.

def _ValidateClosedTimeRange(time_range):
  """Checks that a time-range has both start and end timestamps set."""
  time_range_start, time_range_end = time_range
  _ValidateTimestamp(time_range_start)
  _ValidateTimestamp(time_range_end)
  if time_range_start > time_range_end:
    raise ValueError("Invalid time-range: %d > %d." %
                     (time_range_start.AsMicrosecondsSinceEpoch(),
                      time_range_end.AsMicrosecondsSinceEpoch()))