fix(metrics): add validation/formatting before sending errors to BugSnag

[jackbrewer]

Summary

Fixes #5398

This is my first time working with this repo, so let me know if I've missed anything ☺️

For a while, errors coming from the GitHub API have been in a format not accepted by BugSnag. These appear in the BugSnag dashboard all grouped into a generic notify() received a non-error error. The volume of these is causing issues when looking for actual new errors!

This PR runs the error through some formatting/validation logic. If it already fits one of the expected NotifiableError shapes, we let it through. Non-recognised formats are tweaked to ensure they actually return an Error.

For reference, a valid NotifiableError should match:

export type NotifiableError = Error
  | { errorClass: string, errorMessage: string }
  | { name: string, message: string }
  | string

The objects returned from a GitHub API error are in the unexpected format:

{ message: string, documentation_url: string }

In this case, we throw an error using the message, then append the documentation_url to the event as additional metadata so it's not lost.

Whilst this change won't reduce the volume of errors reported, we'll be able to triage the variations individually (and likely set them back to "handled" like we had done in the past).

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[JGAntunes]

🚀 thank you very much for the 🧹. This looks great 👍