DepotDownloader: added some error handling

--HG--
extra : rebase_source : f849aa26ef59c3eb260b0727ab0f9e7d36700fab

DepotDownloader/CDRManager.cs

@@ -269,6 +269,9 @@ namespace DepotDownloader
             else if ( Util.IsMacOSX() )
                 platformStr = "macos";
 
+            if (appInfoBlob.FileSystems == null)
+                return depotIDs;
+
             foreach ( var blobField in appInfoBlob.FileSystems )
             {
                 string depotPlatform = blobField.Platform;

DepotDownloader/ContentDownloader.cs

@@ -645,6 +645,12 @@ namespace DepotDownloader
                 }
 
                 steam3.RequestDepotKey( ( uint )depotId, ( uint )appId );
+                if (!steam3.DepotKeys.ContainsKey((uint)depotId))
+                {
+                    Console.WriteLine("No valid depot key for {0}, unable to download.", depotId);
+                    return null;
+                }
+
                 byte[] depotKey = steam3.DepotKeys[(uint)depotId];
 
                 var info = new DepotDownloadInfo3( depotId, manifestID, installDir, contentName );