improve(aws): error message when secret access key is missing from credential

[miyamo2]

This PR improves the error message when the awsSecretAccessKey is missing from credentials.
Previously, the error returned was awsAccessKeyID not found, but now it returns awsSecretAccessKey not found, which better matches the actual problem.

For some AWS-related Scaler's tests, the relevant test patterns have already been tested, so no changes have been made.

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

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