Implement `AuthenticationError`

[codebestia]

Description

Raised when the API key is missing, malformed, or rejected by the server (HTTP 401). Should tell the developer exactly what is wrong: missing key, wrong key format, or key revoked.

Proposed Steps

• Define AuthenticationError(ShadeError) in errors.py.
• Carry an optional hint field with corrective guidance (e.g. "Check that you're using a secret key, not a public key").
• The HTTP client raises this on 401 responses.
• Config validation also raises this before any network call.

Acceptance Criteria

• A 401 API response raises AuthenticationError.
• error.hint contains actionable guidance.
• Raised immediately when shade.api_key is None and a resource method is called.
• Subclasses ShadeError.

[Enemuo-debug]

Can I please be opportuned to work on this??

[Elizabeth-dev270]

I enjoy solving software problems and have experience in backend development, APIs, OOP, and scalable application design.