Provide meaningful message if applying /learn to a directory that is empty or only contains unsupported files.

[michaelchia]

Problem

If user applies /learn to a directory that is empty or only contains unsupported files, the message given will be "Learn documents in {load_path} failed. enough values to unpack (expected 2, got 0).".

jupyter-ai/packages/jupyter-ai/jupyter_ai/chat_handlers/learn.py

Line 132 in 3e6b61d

response = f"""Learn documents in **{load_path}** failed. {str(e)}."""

This is not very helpful to the user, especially when it may not be obvious to the user what all the supported extensions are.

Proposed Solution

Provide a more meaningful error response telling the user it could not find any supported files and perhaps indicating the supported extension types.

Additional context