Media: Ensure 'media_handle_upload()' returns 'WP_Error' on failure.

This fixes an issue where failures when inserting the attachment post via
`wp_insert_attachment()` would result in a return value of `0` instead of a
`WP_Error` object, as documented. This is addressed by passing `true` as the 
fourth param (added in WP 4.7.0) when calling `wp_insert_attachment()`.

Props mrasharirfan, flixos90.
Fixes #41726.

Built from https://develop.svn.wordpress.org/trunk@41323


git-svn-id: http://core.svn.wordpress.org/trunk@41154 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/media.php

@@ -378,7 +378,7 @@ function media_handle_upload($file_id, $post_id, $post_data = array(), $override
 	unset( $attachment['ID'] );
 
 	// Save the data
-	$id = wp_insert_attachment($attachment, $file, $post_id);
+	$id = wp_insert_attachment( $attachment, $file, $post_id, true );
 	if ( !is_wp_error($id) ) {
 		wp_update_attachment_metadata( $id, wp_generate_attachment_metadata( $id, $file ) );
 	}

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41322';
+$wp_version = '4.9-alpha-41323';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.