SaveState: Report failed writes, and call back on network refusals

- A failed final flush (a full disk) or a failed rename into place was
  reported as "Saved State". A failed load undo rename still pointed load
  undo at the current game, while the file was another game's state.
- Refusing a save or load for being online didn't call the callback,
  unlike every other refusal.
- A rewind dropped after its decode started stayed active, and stopped
  rewind snapshots until the next clear.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Common/Serialize/Serializer.cpp

@@ -601,6 +601,11 @@ CChunkFileReader::Error CChunkFileReader::SaveFile(const Path &filename, const s
 		INFO_LOG(Log::SaveState, "Savestate: Compressed %i bytes into %i", (int)sz, (int)writeLen);
 	}
 	free(buffer);
+	// Buffered writes can still fail here (a full disk.)
+	if (!pFile.Close() && success) {
+		ERROR_LOG(Log::SaveState, "ChunkReader: Failed closing file");
+		success = false;
+	}
 	if (!success) {
 		return ERROR_BAD_FILE;
 	}