Enhancement - check if the downloaded resources are not corrupted

[radinamatic]

Observed behavior

Experience from building the Funda Wande channel: slow network or the source page may have been causing glitches during the resource download, which in turn resulted in the corrupted PDF files that were unable to open in Kolibri, and several back and forth between the developer cheffing and the QA.

Maybe we could add something to the chef workflow to prevent this type of issues, a step to check that downloaded PDF files are not corrupted and potentially re-trying the download before proceeding to upload to Studio?

[rtibbles]

Interestingly, we don't do any of this validation except implicitly when we happen to look at the file for other purposes (like getting the duration from a video file). I am doing some other work that is adding some of this validation in other places - with that foundation, we should be able to add validation for different specific file types fairly easily.