render LastError on agent gui even when not JSON

[clarkb7]

What does this PR do?

Fix display of go check errors in agent GUI, to match the agent status command

Motivation

Go checks that reported errors only displayed Error: UNKNOWN ERROR in the agent GUI. This happened because Python checks report errors as a JSON string, but Go checks don't.

https://datadoghq.atlassian.net/browse/WINA-521

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

Create an error in a go check, for example, configure the win32_event_log check to monitor an event log that doesn't exist.

Then ensure that agent status command line, agent GUI collector status page, and the dashboard infra page, show the check error.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided. Except if the qa/skip-qa label, with required either qa/done or qa/no-code-change labels, are applied.
• At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• If applicable, the config template has been updated.

[dustmop]

Would be nice if this could get a test, but since there's hardly any here now, I won't block review on that.

[clarkb7]

/merge

[dd-devflow]

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