Why is it in err? What sense does that make? Dagobah is teeming with life, which is a big part of the Force. However, there is also the dark. The place where evil grows and thrives. It's in dark shadows, underground which is often a representation of hell or the underworld in lore. Luke is taking his first steps into the abyss which is the same abyss his father fell into years ago. You're half right. Yoda tells Luke that he doesn't need his weapons. Just himself. The cave isn't fine, it's evil. It's the manifestation of the dark side of the Force. He is going in there to see what his destiny holds for him, though he doesn't know that. He's not projecting, he's sensing the Force itself and he senses the evil coming from there. He feels cold and death. By taking his weapons in there, he will fail because he will rely on his emotion. But even without his weapons, he will still have a vision. By not taking them, he will pass. This is a warning from the Force that he will fail if he goes up against his father now. Just as Anakin's dreams of Padme's death are a warning of what will happen if he continues to seek out power. Just as Anakin's visions of her death keep changing, because while the event is still the same, the details are different. First Obi-wan is not there, then he is, but he has his robe on. When that event comes to pass, Obi-wan is not wearing his robe at all.