fix: keep tiles of other zoom levels drawn in place of failed tiles

[sidlatau]

Fixes #2273.

Since #2166, a tile that fails to load is marked readyToDisplay even without an errorImage, so TileImageView.renderTiles stops keeping the loaded tiles of other zoom levels in its place and the map goes blank, for example when zooming in offline.

A failed tile without errorImage now stays not ready to display, as before #2166, but still notifies its listeners, so tileBuilder is still called with the failed tile. The assertion #2166 removed from _display is back.

The new widget tests cover a failed tile without errorImage (fallback tiles drawn), with tileBuilder (builder gets the failed tile, fallback tiles drawn), and with errorImage (error image drawn, unchanged).