Make it easier to debug .obj file parsing errors

internal/data/object.go

@@ -92,7 +92,7 @@ func LoadObject(filename string) (*Object, error) {
 
 	out := &Object{Filename: filename}
 	if err := binary.Read(f, binary.LittleEndian, &out.ObjectHeader); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Reading object header: %v", err)
 	}
 
 	if err := out.ObjectHeader.Check(); err != nil {
@@ -102,30 +102,30 @@ func LoadObject(filename string) (*Object, error) {
 	// Now load all sprites into memory
 	dir := make([]dirEntry, out.NumSprites)
 	if _, err := f.Seek(int64(out.DirOffset), io.SeekStart); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Seeking to sprite directory: %v", err)
 	}
 
 	if err := binary.Read(f, binary.LittleEndian, &dir); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Reading sprite directory: %v", err)
 	}
 
 	if _, err := f.Seek(int64(out.DataOffset), io.SeekStart); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Seeking to sprites: %v", err)
 	}
 
-	for _, dirEntry := range dir {
+	for i, dirEntry := range dir {
 		if err := dirEntry.Check(); err != nil {
 			return nil, err
 		}
 
 		if _, err := f.Seek(int64(out.DataOffset+dirEntry.Offset), io.SeekStart); err != nil {
-			return nil, err
+			return nil, fmt.Errorf("Seeking to sprite %v: %v", i, err)
 		}
 
 		sprite := &Sprite{}
 
 		if err := binary.Read(f, binary.LittleEndian, &sprite.SpriteHeader); err != nil {
-			return nil, err
+			return nil, fmt.Errorf("Reading sprite %v header: %v", i, err)
 		}
 
 		if err := sprite.Check(dirEntry.Size); err != nil {
@@ -140,7 +140,7 @@ func LoadObject(filename string) (*Object, error) {
 
 		for y := 0; y < int(sprite.Height); y++ {
 			if row, err := buf.ReadBytes(0x00); err != nil {
-				return nil, err
+				return nil, fmt.Errorf("Reading row %v for sprite %v: %v", y, i, err)
 			} else {
 				sprite.Rows[y] = row
 			}