check for SIGUSR1 before using it, closes #3074

thanks @takluyver for spotting this

IPython/frontend/html/notebook/notebookapp.py

@@ -550,6 +550,8 class NotebookApp(BaseIPythonApplication):
             # but it will work
             signal.signal(signal.SIGINT, self._handle_sigint)
         signal.signal(signal.SIGTERM, self._signal_stop)
+        if hasattr(signal, 'SIGUSR1'):
+            # Windows doesn't support SIGUSR1
         signal.signal(signal.SIGUSR1, self._signal_info)
         if hasattr(signal, 'SIGINFO'):
             # only on BSD-based systems