Always throw a stack trace if an invalid tile is requested, so that we can more easily track down these errors

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3603 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/media/tile/TileSet.java

@@ -327,6 +327,7 @@ public abstract class TileSet
         } else {
             Log.warning("Requested invalid tile [tset=" + this +
                         ", index=" + tileIndex + "].");
+            Thread.dumpStack();
             return false;
         }
     }