fix(datadog): Non-map values for error key

[giddie]

Passing an error key is a really easy footgun:

Logger.error("Something bad happened", error: "{:error, :something_bad}")

Currently, the formatter crashes because it expects error to be a map. This PR fixes the issue by assuming the caller was not trying to define the error map, and instead nests the value inside the error map:

%{
  error: %{
    kind: "error",
    message: "Something bad happened",
    error: "{:error, :something_bad}"
  }
}

[maximejimenez]

Hello @AndrewDryga
We are currently experiencing a similar issue, this would be a great improvement to prevent the formatter crashing 🙏
Thank you for your time!

[coveralls]

coverage: 100.0%. remained the same
when pulling 495b917 on giddie:master
into 1524672 on Nebo15:master.

[AndrewDryga]

Thank you ❤️