feat: log a warning when authentication with APM server fails

[kruskall]

The intake route of the APM server sends a 202 status accepted
if the request succeeds.

See https://github.com/elastic/apm-server/blob/main/docs/api-events.asciidoc#response

If auth fails the server returns a 401.

See https://github.com/elastic/apm-server/blob/3cd1a63bf218ea0ab0f240924236823e1815882b/beater/middleware/auth_middleware.go#L54-L57 and https://github.com/elastic/apm-server/blob/3cd1a63bf218ea0ab0f240924236823e1815882b/beater/request/result.go#L86

Related to #205

[ghost]

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• Start Time: 2022-07-13T14:52:54.237+0000

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