Gracefully handle seekability errors during frame reading

[gitttt-1234]

In #129 we added basic valid frame index validation to throw IndexErrors on trying to read frames that are beyond the number of frames reported by the backend.

As we know however, some video container formats don't produce reliably frame-accurate seeking, so seeking to the end of the video may not work.

We should catch these cases for the opencv and imageio-ffmpeg backends and raise an appropriate error. Right now I think the opencv backend just returns a None, which leads to a weird call stack that can be hard to debug (this is why we added the basic index validation in the first place).