Display media upload error message from server when one exists

[danielbachhuber]

Description

If the server rejects a media upload and includes a message, display that message in the error UI.

Requires #7386
Fixes #7137

How has this been tested?

Add the following code snippet in functions.php or similar:

$force_upload_failure = function( $file ) {
	$file['error'] = 'This file upload has been forced to fail.';
	return $file;
};
add_filter( 'wp_handle_sideload_prefilter', $force_upload_failure );
add_filter( 'wp_handle_upload_prefilter', $force_upload_failure );

Attempt to upload an image into the image block. Observe This file upload has been forced to fail. as the error message.

Screenshots

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.