Fix crash if a save thumbnail is missing from the server

Which basically never happens. Except when the server runs out of space :) See 0f418ec195 for a similar fix.

src/gui/interface/SaveButton.cpp

@@ -148,7 +148,14 @@ void SaveButton::Tick(float dt)
 
 		if (thumbnailRequest && thumbnailRequest->CheckDone())
 		{
-			thumbnail = thumbnailRequest->Finish();
+			try
+			{
+				thumbnail = thumbnailRequest->Finish();
+			}
+			catch (const http::RequestError &ex)
+			{
+				// TODO: handle
+			}
 			thumbnailRequest.reset();
 		}