Added log message when a plugin cancels event submission

Author: niemyjski

src/Exceptionless/ExceptionlessClient.cs

@@ -185,8 +185,10 @@ public void SubmitEvent(Event ev, ContextData pluginContextData = null) {
 
             var context = new EventPluginContext(this, ev, pluginContextData);
             EventPluginManager.Run(context);
-            if (context.Cancel)
+            if (context.Cancel) {
+                _log.Value.FormattedInfo(typeof(ExceptionlessClient), "Event submission cancelled by a plugin: id={0} type={1}", ev.ReferenceId, ev.Type);
                 return;
+            }
 
             // ensure all required data
             if (String.IsNullOrEmpty(ev.Type))