Add error details to autoscaling backoff.

[walidghallab]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds more detailed error information to backoff interface & implementation.
This is needed as it will allow getting this info easily to show them as a reason for backoff.

Which issue(s) this PR fixes:

#6318

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: walidghallab (cf6176f)

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[towca]

Everything apart from the 2 structs I mention in one comment LGTM, just some minor comments. Still need to understand the structs

[towca]

Thanks for addressing the comments!
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: towca, walidghallab

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