fix(ecr): change log level of non-scanned images

[sergargar]

Description

Change log level of non-scanned images to WARNING since it is expected for images that have not been scanned:

ScanNotFoundException[172]: An error occurred (ScanNotFoundException) when calling the DescribeImageScanFindings operation: Image scan does not exist

Checklist

• Are there new checks included in this PR? Yes / No
  • If so, do we need to update permissions for the provider? Please review this carefully.
• [x] Review if the code is being covered by tests.
• [x] Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.09%. Comparing base (ae1b114) to head (a60fc7c). Report is 8 commits behind head on master.

Files	Patch %	Lines
prowler/providers/aws/services/ecr/ecr_service.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4747 +/- ## ======================================= Coverage 89.09% 89.09% ======================================= Files 920 920 Lines 28125 28128 +3 ======================================= + Hits 25057 25062 +5 + Misses 3068 3066 -2 ```

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[github-actions[bot]]

💚 All backports created successfully

Status	Branch	Result
✅	v3
✅	v4.3

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