I get an error when I try and download a movie

The movie I was downloading was part of a digital copy that I received when I bought the blu ray disc of Star Trek Into the Darkness. I was able to download the movie, but when it tried to download the iExtras file it only downloaded half of it and kept on giving me a "stop err" message. I tried several times and it all failed. I deleted it from my queue to download.

Since this started downloading automatically, how can I restart downloading it without having to "buy" it?

This is what the error message said:

There were problems downloading some purchased items.

For more information on the items that could not be downloaded, click below.

There was a problem downloading “Star Trek Into Darkness - iTunes Extras / Star Trek Into Darkness / J.J. Abrams”. An unknown error occurred (-50).

Please check that the connection to the network is active and try again.

There was a problem downloading “Star Trek Into Darkness - iTunes Extras - Supplemental Video #1 / J.J. Abrams”. An unknown error occurred (-50).

Please check that the connection to the network is active and try again.

There was a problem downloading “Star Trek Into Darkness - iTunes Extras / Star Trek Into Darkness / J.J. Abrams”. An unknown error occurred (-50).

Please check that the connection to the network is active and try again.