out_cloudwatch_logs: enhance api error response readability

[matthewfala]

Tested by adding a fake error:

[2023/11/02 20:43:59] [ warn] [output:cloudwatch_logs:cloudwatch_logs.1] PutLogEvents: Raw response: {"nextSequenceToken":"49645959017322090641208425572396895883538558252795036370"}

It works.

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Testing
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Backporting

• Backport to latest stable release.

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