Changed Akka.Streams backoff error logging to include full exception …

…message (#4472)

* Changed Akka.Streams backoff error logging to include full exception message

* Use dedicated warning message for exceptions

src/core/Akka.Streams/Dsl/Restart.cs

@@ -494,7 +494,7 @@ protected SubSinkInlet<TOut> CreateSubInlet(Outlet<TOut> outlet)
                         Fail(Out, ex);
                     else
                     {
-                        Log.Warning($"Restarting graph due to failure. Stacktrace: {ex.StackTrace}");
+                        Log.Warning(ex, "Restarting graph due to failure.");
                         ScheduleRestartTimer();
                     }
                 }));