Improve Error Handling

[JensAstrup]

Some API calls silently ignore failures. Error handling should be consistent and informative.

Proposed Solution:

•	Create custom exception hierarchy (OutlookAPIError, AuthenticationError, RateLimitError, etc.)
•	Centralize error response parsing in HTTP client
•	Always raise on non-2xx responses (unless explicitly documented otherwise)
•	Include response body in error messages for debugging

[linear]

PYO-10