python: fix get_property error when property doesn't exist

In Python bindings, Message.get_property fails with an AttributeError when trying to fetch a property that doesn't exist.

From d712832ba982085975c27b23bb502af82e638b39 Mon Sep 17 00:00:00 2001
From: hydrargyrum <dev@indigo.re>
Date: Sun, 6 Jan 2019 16:08:55 +0100
Subject: [PATCH] python: fix get_property error when property doesn't exist