It looks like you managed to update in the few minutes between commits. Chris and I co-author this plugin and our deploy method uses grunt-wp-deploy to simplify things, but I noticed it doesn’t always push new files to the SVN.
In version 4.0.0, two new files were added: validation.php and tag-generator.min.css. Because of the issue with grunt deploy, I had to manually commit them to the repo after (see manual changesets for trunk and tag compared to these automated ones for trunk and tag).
If your update took place before the files were added, then you’ll see this error saying that the file does not exist. Even though the changesets are about 4 minutes apart, that’s still enough time for errors like this to happen if updating at just the right time.
To remedy, I suggest deleting the plugin and re-installing it fresh from the repo now that the files are all there. Deleting/un-installing the plugin won’t delete any form data, so once it’s re-installed and activated, all will be well.
Alternatively, if you’re familiar with FTP, you can download the plugin zip file, unzip it, and manually add those two files into version 4.0.0 of your broken version.
Sorry your update was such a mess!