Fix files never downloading (unless it has been downloaded in the past
and using `!` force flag).
The logic should be:
* If file does not exist, or `!` force flag used, then download it
* Else if neither `!` or `:open` flag used, then error out
and then return downloaded-message or open-and-message.
I.e. `:open` should still open the file if it has already been
downloaded. Otherwise the only way to open it is to use `!` and
re-download it.
Benjamin Große
8eef8787cc