bindings: (fixes #796) Gracefully exit when bindings are unavailable

examples/tutorial/third.py

@@ -16,7 +16,14 @@
 #  * Ported to Python by Mohit P. Tahiliani
 #  */
 
-from ns import ns
+try:
+    from ns import ns
+except ModuleNotFoundError:
+    raise SystemExit(
+        "Error: ns3 Python module not found;"
+        " Python bindings may not be enabled"
+        " or your PYTHONPATH might not be properly configured"
+    )
 import sys
 
 # // Default Network Topology