Scenario viewpoint, Z index management, and arrow controls

internal/assetstore/map.go

@@ -1,6 +1,7 @@
 package assetstore
 
 import (
+	"fmt"
 	"image"
 	"log"
 
@@ -95,7 +96,7 @@ func (m *Map) SpritesForCell(x, y, z int) ([]*Sprite, error) {
 
 		obj, err := m.set.Object(ref.Index())
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("Failed to get object for %#+v: %v", ref, err)
 		}
 
 		sprite, err := obj.Sprite(ref.Sprite())