Source

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

public function feedback( $data, ...$args ) {
		if ( is_wp_error( $data ) ) {
			$string = $data->get_error_message();
		} elseif ( is_array( $data ) ) {
			return;
		} else {
			$string = $data;
		}
		if ( ! empty( $this->upgrader->strings[ $string ] ) ) {
			$string = $this->upgrader->strings[ $string ];
		}

		if ( strpos( $string, '%' ) !== false ) {
			if ( ! empty( $args ) ) {
				$string = vsprintf( $string, $args );
			}
		}

		$string = trim( $string );

		// Only allow basic HTML in the messages, as it'll be used in emails/logs rather than direct browser output.
		$string = wp_kses(
			$string,
			array(
				'a'      => array(
					'href' => true,
				),
				'br'     => true,
				'em'     => true,
				'strong' => true,
			)
		);

		if ( empty( $string ) ) {
			return;
		}

		$this->messages[] = $string;
	}