Make sigterm listen optional (fixes macOS problem)

zeronet.py

@@ -12,8 +12,10 @@ def main():
     main = None
     try:
         import signal
-
+        try:
             signal.signal(signal.SIGTERM, lambda signum, stack_frame: sys.exit(0))
+        except Exception as err:
+            print("Error setting up SIGTERM watcher: %s" % err)
 
         app_dir = os.path.dirname(os.path.abspath(__file__))
         os.chdir(app_dir)  # Change working dir to zeronet.py dir