Add more checks for video seeking/reading failure (#138)

sleap_io/io/video_reading.py

@@ -397,16 +397,21 @@ def _read_frame(self, frame_idx: int) -> np.ndarray:
             This does not apply grayscale conversion. It is recommended to use the
             `get_frame` method of the `VideoBackend` class instead.
         """
+        failed = False
         if self.plugin == "opencv":
             if self.reader.get(cv2.CAP_PROP_POS_FRAMES) != frame_idx:
                 self.reader.set(cv2.CAP_PROP_POS_FRAMES, frame_idx)
-            _, img = self.reader.read()
+            success, img = self.reader.read()
         elif self.plugin == "pyav" or self.plugin == "FFMPEG":
             if self.keep_open:
                 img = self.reader.read(index=frame_idx)
             else:
                 with iio.imopen(self.filename, "r", plugin=self.plugin) as reader:
                     img = reader.read(index=frame_idx)
+
+        success = (not failed) and (img is not None)
+        if not success:
+            raise IndexError(f"Failed to read frame index {frame_idx}.")
         return img
 
     def _read_frames(self, frame_inds: list) -> np.ndarray: