fix(api): raise HTTPError instead of returning None on API errors

[MD-Mushfiqur123]

Resolves #820

Changes:

• Replaced _log_error() + return None pattern with r.raise_for_status() in all ApiClient methods
• Removed unused _log_error method
• Removed None-check in check_organization_exists (method now raises on HTTP errors)
• add_emission and _create_run already have try/except wrappers so they continue to handle errors gracefully (logging + returning False/None)
• CodeCarbonAPIOutput (used by EmissionsTracker) already wraps add_emission in try/except
• Updated tests to expect requests.HTTPError instead of None returns