Exit with 255 since that's more explicit.

Rationale behind exit(-1) is that it turns into highest available
number which hopefully won't collide with a wrapping shell script
trying to exit with different codes depending on error (like 1, 2, 3).

lib/chutney/Traffic.py

@@ -275,7 +275,7 @@ def main():
 
     if success:
         return 0
-    return -1
+    return 255
 
 if __name__ == '__main__':
     sys.exit(main())