[backend] Change log level for S3 download file errors (#7467)

[aHenryJard]

Proposed changes

• Change log level to error when S3 throw error + add another log in case the response body from S3 is null to have different log in the 2 different situations.

I did not change the behavior (like throwing exception instead of just log it) because it's use at many places and I don't want to change it all. I just made the current behavior explicit in tests.

Related issues

Closes 7467

Checklist

• [x] I consider the submitted work as finished
• [x] I tested the code for its functionality
• [x] I wrote test cases for the relevant uses case (coverage and e2e)
• [x] I added/update the relevant documentation (either on github or on notion)
• [x] Where necessary I refactored code to improve the overall quality

Further comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.37%. Comparing base (8590220) to head (6a8701f). Report is 2 commits behind head on master.

Files	Patch %	Lines
...tform/opencti-graphql/src/database/file-storage.js	70.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #7468 +/- ## ======================================= Coverage 67.36% 67.37% ======================================= Files 563 563 Lines 69643 69652 +9 Branches 5886 5888 +2 ======================================= + Hits 46916 46927 +11 + Misses 22727 22725 -2 ```

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