Source

File: wp-admin/includes/class-wp-upgrader-skin.php

public function error( $errors ) {
		if ( ! $this->done_header ) {
			$this->header();
		}
		if ( is_string( $errors ) ) {
			$this->feedback( $errors );
		} elseif ( is_wp_error( $errors ) && $errors->has_errors() ) {
			foreach ( $errors->get_error_messages() as $message ) {
				if ( $errors->get_error_data() && is_string( $errors->get_error_data() ) ) {
					$this->feedback( $message . ' ' . esc_html( strip_tags( $errors->get_error_data() ) ) );
				} else {
					$this->feedback( $message );
				}
			}
		}
	}