Revert "Remove existence check for TmpTransferDir"

Since we're now back on using a central location for storing temporary
transfer files, it does make sense again to check for that location to
exist on service startup (refers to #16).

This reverts commit 10640754.

ATxCommon/Serializables/ServiceConfig.cs

@@ -403,6 +403,11 @@ namespace ATxCommon.Serializables
             if (!Directory.Exists(c.DestinationDirectory))
                 errmsg += $"can't find (or reach) destination: {c.DestinationDirectory}\n";
 
+            // TmpTransferDir
+            var tmpTransferPath = Path.Combine(c.DestinationDirectory, c.TmpTransferDir);
+            if (!Directory.Exists(tmpTransferPath))
+                errmsg += $"can't find (or reach) temporary transfer dir: {tmpTransferPath}\n";
+
 
             ////////// OPTIONAL PARAMETERS SETTINGS VALIDATION //////////