don't post 'operation aborted' UDP errors when changing listen port

ChangeLog

@@ -89,6 +89,7 @@
 	  incoming connection
 	* added more detailed instrumentation of the disk I/O thread
 
+	* don't post 'operation aborted' UDP errors when changing listen port
 	* fix tracker retry logic, where in some configurations the next tier would not be tried
 	* fixed bug in http seeding logic (introduced in 0.15.7)
 	* add support for dont-have extension message

src/session_impl.cpp

@@ -2089,7 +2089,9 @@ namespace aux {
 					m_stat.received_tracker_bytes(len + 28);
 			}
 
-			if (m_alerts.should_post<udp_error_alert>())
+			// don't bubble up operation aborted errors to the user
+			if (e != asio::error::operation_aborted
+				&& m_alerts.should_post<udp_error_alert>())
 				m_alerts.post_alert(udp_error_alert(ep, e));
 			return;
 		}