[bug 1116489] Tweak ImportError message in manage.py

If they have a virtual environment activated, then the problem is
probably either they haven't installed the requirements or they haven't
updated them in a while.

If they don't have a virtual environment actiivated, then they need to
activate it.

This code change attempts to distinguish between the two by looking for
VIRTUAL_ENV in the environment.

manage.py

@@ -18,7 +18,10 @@ except ImportError:
     print 'OH NOES! There was an import error:'
     print ''
     print ''.join(traceback.format_exception(*sys.exc_info()))
-    print 'Have you activated your virtual environment?'
+    if 'VIRTUAL_ENV' in os.environ:
+        print 'Have you installed requirements? Are they up-to-date?'
+    else:
+        print 'Have you activated your virtual environment?'
     sys.exit(1)
 
 # Import for side-effect: configures our logging handlers.