fix(spec): render response body for non-200 responses

[glowcloud]

Refs #9556

[char0n]

I'm wondering why we use serializeError when creating payloads.
Flux Standard Action doesn't have any limitation over what can be attached to the action payload. Only reducers when and if reducing errors, should possibly use serializeError.
I don't see any reason why we should serialize the error, if you drop the serialization, the issue will be resolved, without any other change, right?

[char0n]

Looks good! We can further simplify the change by not using serializeError for flux action payloads.

[char0n]

Great job!