How do I get meaningful error messages from a view page in an MVC app?

Question

I have this boilerplate code in one of my views in an MVC app:

<form enctype="multipart/form-data" method="post">
    <dl>
        <dt>
            <label asp-for="FileUpload.FormFile"></label>
        </dt>
        <dd>
            <input asp-for="FileUpload.FormFile" type="file">
            <span asp-validation-for="FileUpload.FormFile"></span>
        </dd>
    </dl>
    <input asp-page-handler="Upload" class="btn" type="submit" value="Upload" />
</form>  

For some weird reason, it's only uploading about half of the files that I've selected. I run the app in a Docker container, upload a file, close the app, reopen it, upload a different file, and it fails. The files that fail, fail consistently, and the files that succeed, succeed consistently. I know that the error is taking place within the view and specifically in the form I posted above. I've debugged the crap out of the controller and nothing is failing in my C# backend code. How do I get a meaningful error from the view?