Isolated chunk update failures

- Treats only loading saved biome information as an engine failure
- Leaves a container without loot and warns once when its saved biome cannot be recovered
- Keeps a failed chunk unetched and retries it without stopping the rest of the level's update pass
- Ends a level's pass only when its engine or mantle is closed
- Covers the deferred, permanent and isolated update paths through updateChunk and the server tick

core/src/test/java/art/arcane/iris/generation/runtime/GenerationFailuresTest.java

@@ -34,6 +34,14 @@ public class GenerationFailuresTest {
         assertFalse(GenerationFailures.isEngineFailure(new IllegalStateException("unrelated")));
     }
 
+    @Test
+    public void permanentSavedBiomeUnavailabilityIsAContentFailure() {
+        assertFalse(GenerationFailures.isEngineFailure(new SavedBiomeUnavailableException(
+                "This chunk has no exact saved Iris biome assignment for generation 3.", false)));
+        assertFalse(GenerationFailures.isEngineFailure(new SavedBiomeUnavailableException(
+                "Unable to read the saved biome at chunk 0,0.", new IllegalArgumentException("bad header"))));
+    }
+
     @Test
     public void anyFailureOnAnInterruptedThreadIsAnEngineFailure() {
         Thread.currentThread().interrupt();