Display error when image fails to upload and server returns error message

[danielbachhuber]

In the WordPress Media Library, a failed image upload will display the specific error returned by the server:

However, Gutenberg displays a generic "Error while uploading" message:

It would be better to display the specific error message.

This scenario can be reproduced with the following code snippet:

$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 );

Tested with Gutenberg 3.0.

[danielbachhuber]

This might need to wait until #6968 lands to avoid refactor conflict.