genPOI: catch ChunkDoesntExist when processes > 1

Fixes #1480.

No clue how this wasn't an issue before but is an issue now in 1.13,
maybe Minecraft writes more placeholder chunks these days.

overviewer_core/aux_files/genPOI.py

@@ -127,6 +127,8 @@ def parseBucketChunks((bucket, rset, filters)):
                         markers[name].append(d)
         except nbt.CorruptChunkError:
             logging.warning("Ignoring POIs in corrupt chunk %d,%d", b[0], b[1])
+        except world.ChunkDoesntExist:
+            pass
 
         # Perhaps only on verbose ?
         i = i + 1