Just for people to know how I've fixed the non-existent file page. It took 2 steps.
1) Create a new phrase as per the image:
2) Changed the code on line 672 as follows:
from
PHP Code:
echo "<html><head><title>DownloadsII Error</title></head><body>ERROR: File not found.</body></html>";
to
PHP Code:
eval(standard_error(fetch_error('dl2_file_not_found', 'downloads.php?do=report&fileid=' . $file['id'])));
A similar phrase and code replacement will also work for when the filename is the empty string.