text: Title: `default.faiss` not checked for existance before using
  Body: jupyter-ai seems not to check if `default.faiss` exists before using it through `faiss`
  ```
  [E 2024-08-03 17:49:43.118 AiExtension] Could not load vector index from disk. Full exception details printed below.
  [E 2024-08-03 17:49:43.118 AiExtension] Error in faiss::FileIOReader::FileIOReader(const char*) at /project/faiss/faiss/impl/io.cpp:67: Error: 'f' failed: could not open /p/home/user1/cluster1/.local/share/jupyter/jupyter_ai/indices/default.faiss for reading: No such file or directory
  ```
  and it also does not regenerate it if the file is missing.
  
  (I am using jupyter-ai 2.19.1 in combination with JupyterLab 4.2.1)
  