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>

Core/SaveStateRewind.cpp

@@ -161,6 +161,16 @@ CChunkFileReader::Error StateRingbuffer::Restore(std::string *errorString, std::
 	return FinishRestore(errorString, metadata);
 }
 
+void StateRingbuffer::CancelRestore() {
+	if (!restoreActive_) {
+		return;
+	}
+	// The decode may still be running into restoreBuffer_.
+	WaitForWorker();
+	restoreActive_ = false;
+	std::vector<u8>().swap(restoreBuffer_);
+}
+
 // Only moves the ring back: the state's buffer may still be in use by a worker job, and is reused
 // by the next snapshot anyway.
 void StateRingbuffer::DropNewest() {