Explain what Sage says to Clary and Lina at the greenhouse. What questions does this bring to Lina's mind in Chapter 4?

Sadge tells Clary and Lina....

“Nothing! Nothing! There is nothing out there!” His voice became a shout and his eyes looked wild again. “Or if there is, we can never get to it. Never! Not without a light.” He took a long, shaky breath. For a while he stared at the floor. Then he stood up. “I think I’m all right now. I’ll be going.”

This leads Lina to wonder why "Everyone who had tried to penetrate the Unknown Regions had come back within a few hours, their enterprise a failure."