Show proper error message in diff when failing to load image

kittens/diff/render.go

@@ -362,7 +362,7 @@ func image_lines(left_path, right_path string, screen_size screen_size, margin_s
 		if errors.Is(err, graphics.ErrNotFound) {
 			return splitlines("Loading image...", available_cols)
 		}
-		return splitlines(fmt.Sprintf("Failed to load image: %s", err), available_cols)
+		return splitlines(fmt.Sprintf("%s", err), available_cols)
 	}
 	left_lines := do_side(left_path)
 	if ll.left_image.count = len(left_lines); ll.left_image.count > 0 {

tools/tui/graphics/collection.go

@@ -86,6 +86,9 @@ func (self *ImageCollection) GetSizeIfAvailable(key string, page_size Size) (Siz
 	}
 	ans := img.renderings[page_size]
 	if ans == nil {
+		if img.err != nil {
+			return Size{}, img.err
+		}
 		return Size{}, ErrNotFound
 	}
 	return Size{ans.img.Width, ans.img.Height}, img.err