Upload error in UI should specifically convey if a status 413 was received

[leedxw]

In the event that a user attempts to upload a media file that is below the maximum php filesize, but where the POST is above the limit set by the web server, the error message (en) is:

Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page.

(It's a common configuration in web servers to set the max post/body sizes to be the same value as the maximum file size, but - since there's an overhead - files that appear to be just below the advertised limit will fail to upload.)

When /wp-admin/async-upload.php receives a 413 code the user presented error message should include this code and the explanation - such as "413 Request Entity Too Large" (en) - since this is the information that will be useful for whoever does technical support for the site.