tools: present nicer error for missing packaging

This makes the error more user friendly and suggests what to do to fix
it.

msg/tools/px_generate_uorb_topic_files.py

@@ -43,7 +43,6 @@ import shutil
 import filecmp
 import argparse
 import sys
-from packaging import version
 
 try:
     import em
@@ -65,6 +64,16 @@ except ImportError as e:
     print("")
     sys.exit(1)
 
+try:
+    from packaging import version
+except ImportError as e:
+    print("Failed to import packaging: " + str(e))
+    print("")
+    print("You may need to install it using:")
+    print("    pip3 install --user packaging")
+    print("")
+    sys.exit(1)
+
 
 __author__ = "Sergey Belash, Thomas Gubler, Beat Kueng"
 __copyright__ = "Copyright (C) 2013-2016 PX4 Development Team."