fix: raise HTTP errors in ApiClient instead of returning None (#820)

[agodianel]

Resolves #820 by raising requests.exceptions.HTTPError when the API returns a non-success status code, rather than failing silently and returning None. This allows the EmissionsTracker to catch the exception and fall back to offline mode without crashing the user's execution run.

Description

Fixes #820

The ApiClient previously returned None or False on HTTP errors, which failed silently for the user but occasionally crashed upstream logic (e.g. attempting to read api.run_id when it was None). As requested in the issue, this PR updates the client to throw requests.exceptions.HTTPError on bad status codes and handles the exception upstream to fall back gracefully.

Changes:

• Replaced return None/False/[] with r.raise_for_status() in all ApiClient data methods.
• Re-raised exceptions in generic try...except blocks within ApiClient (e.g., inside _create_run).
• Wrapped CodeCarbonAPIOutput initialization in emissions_tracker.py with a try...except block so that if the API is unreachable, the execution falls back to local tracking instead of tearing down the user's workload.
• Added test_call_api_error_raises to test_api_call.py to ensure that 500 error responses raise the correct HTTP error natively.

All local CodeCarbon unit tests (including the new ones) execute and pass successfully.

[codecov]

Codecov Report

❌ Patch coverage is 13.04348% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.87%. Comparing base (46ac6dd) to head (ac1dba7).

Files with missing lines	Patch %	Lines
codecarbon/core/api_client.py	18.75%	13 Missing ⚠️
codecarbon/emissions_tracker.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
+ Coverage   80.74%   80.87%   +0.13%     
==========================================
  Files          41       41              
  Lines        3921     3927       +6     
==========================================
+ Hits         3166     3176      +10     
+ Misses        755      751       -4     

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