Missing sys import

Also put the error message in sys.exit rather than printing. This will output it to stderr and make the exit code non-zero
like it should be.

lib/websockets.py

@@ -21,8 +21,8 @@ from collections import defaultdict
 try:
     from SimpleWebSocketServer import WebSocket, SimpleSSLWebSocketServer
 except ImportError:
-    print "install SimpleWebSocketServer"
+    import sys
-    sys.exit()
+    sys.exit("install SimpleWebSocketServer")
 
 import util