Coding Standards: Rename the $errorString variable in wp_xmlrpc_server methods.

This resolves a WPCS warning:
{{{
Variable "$errorString" is not in valid snake_case format, try "$error_string"
}}}

Follow-up to [5054].

See #62279.

git-svn-id: https://develop.svn.wordpress.org/trunk@59672 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-xmlrpc-server.php

@@ -6434,9 +6434,10 @@ public function mw_newMediaObject( $args ) {
 		$upload = wp_upload_bits( $name, null, $bits );
 		if ( ! empty( $upload['error'] ) ) {
 			/* translators: 1: File name, 2: Error message. */
-			$errorString = sprintf( __( 'Could not write file %1$s (%2$s).' ), $name, $upload['error'] );
-			return new IXR_Error( 500, $errorString );
+			$error_string = sprintf( __( 'Could not write file %1$s (%2$s).' ), $name, $upload['error'] );
+			return new IXR_Error( 500, $error_string );
 		}
+
 		// Construct the attachment array.
 		$post_id = 0;
 		if ( ! empty( $data['post_id'] ) ) {
@@ -6446,6 +6447,7 @@ public function mw_newMediaObject( $args ) {
 				return new IXR_Error( 401, __( 'Sorry, you are not allowed to edit this post.' ) );
 			}
 		}
+
 		$attachment = array(
 			'post_title'     => $name,
 			'post_content'   => '',