Add missing return when datafixers fail for chunk conversion

Caused the chunk to be later deserialized, which would
result in spamming unrelated errors (possibly) +
double chunk completion.

patches/server/Asynchronous-chunk-IO-and-loading.patch

@@ -1544,6 +1544,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +            } catch (final Throwable ex) {
 +                PaperFileIOThread.LOGGER.error("Could not apply datafixers for chunk task: " + this.toString(), ex);
 +                this.complete(ChunkLoadTask.createEmptyHolder());
++                return;
 +            }
 +
 +            if (this.checkCancelled()) {