[core] don't mark tiles with failed requests as obsolete

Otherwise, this will remove them from the map and overwrite the old data. This could happen e.g. when we have connection trouble and a tile needs to be refreshed, or we're displaying stale information and the connection is down.

src/mbgl/map/source.cpp

@@ -531,7 +531,7 @@ void Source::tileLoadingCompleteCallback(const TileID& tileID, const TransformSt
         return;
     }
 
-    if (tileData->getState() == TileData::State::obsolete && tileData->getError()) {
+    if (tileData->hasError()) {
         observer->onTileError(*this, tileID, tileData->getError());
         return;
     }