Add listen for errors sample. [(#1306)](GoogleCloudPlatform/python-docs-samples#1306)

* Add listen for errors sample.

* Update subscriber.py

* Update subscriber.py

Author: noerog

samples/snippets/subscriber.py

@@ -110,6 +110,30 @@ def callback(message):
         time.sleep(60)
 
 
+def listen_for_errors(project, subscription_name):
+    """Receives messages and catches errors from a pull subscription."""
+    subscriber = pubsub_v1.SubscriberClient()
+    subscription_path = subscriber.subscription_path(
+        project, subscription_name)
+
+    def callback(message):
+        print('Received message: {}'.format(message))
+        message.ack()
+
+    subscription = subscriber.subscribe(subscription_path, callback=callback)
+
+    # Blocks the thread while messages are coming in through the stream. Any
+    # exceptions that crop up on the thread will be set on the future.
+    future = subscription.open(callback)
+    try:
+        future.result()
+    except Exception as e:
+        print(
+            'Listening for messages on {} threw an Exception: {}.'.format(
+                subscription_name, e))
+        raise
+
+
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(
         description=__doc__,
@@ -143,6 +167,10 @@ def callback(message):
         help=receive_messages_with_flow_control.__doc__)
     receive_with_flow_control_parser.add_argument('subscription_name')
 
+    listen_for_errors_parser = subparsers.add_parser(
+        'listen_for_errors', help=listen_for_errors.__doc__)
+    listen_for_errors_parser.add_argument('subscription_name')
+
     args = parser.parse_args()
 
     if args.command == 'list_in_topic':
@@ -160,3 +188,5 @@ def callback(message):
     elif args.command == 'receive-flow-control':
         receive_messages_with_flow_control(
             args.project, args.subscription_name)
+    elif args.command == 'listen_for_errors':
+        listen_for_errors(args.project, args.subscription_name)