Fix download_media to error out more cleanly

[bctcvai]

Some legacy code should be removed / changed to make the error condition of a Media record having no underlying content more obvious.

https://github.com/cvisionai/tator-py/blob/dc6ab1552bdf2a902667fd6b4a639790fef85fb8/tator/util/download_media.py#L107-L112

Probably just raise an exception, the referenced code doesn't work with tator-py bindings.