Handle build download errors in update checker. Fixes issue #2512.

source/updatecheck.pas

@@ -235,6 +235,7 @@ begin
   if FileExists(Download.Filename) then
     DeleteFile(Download.Filename);
 
+  try
     // Do the download
     Download.ExecuteTarget(nil);
 
@@ -265,6 +266,10 @@ begin
         Stream.Free;
       end;
     end;
+  except
+    on E:Exception do
+      ErrorDialog(E.Message);
+  end;
 end;