(#4922) Don't truncate remotely-sourced files on 404

We were 'handling' 404's on remote file content retrieval by returning nil
rather than raising an exception. This caused no content to be written to the
temporary file, but still appeared successful, so the destination file was
overwritten, instead of preserved. Now we just handle 404 like any other
error.

Note that the root cause of these 404s seems to have been #4319, which has been
fixed. However, in the event we do happen to get a 404 here, it's better not to
have code to specifically handle it incorrectly.

Paired-With: Max Martin
Reviewed-By: Matt Robinson