nicer error message if python-nose is not installed

tests/run_tests.py

@@ -1,5 +1,13 @@
 #!/usr/bin/env python
 
+import sys
+
+try:
+    import nose
+except ImportError:
+    sys.stderr.write("Unable to run python tests: the third party 'nose' module is required\nTo install 'nose' do:\n\tsudo pip install nose (or on debian systems: apt-get install python-nose\n")
+    sys.exit(1)
+    
 from python_tests.utilities import TodoPlugin
 from nose.plugins.doctests import Doctest