Multiple error handler labels

[johakoch]

config.ErrorHandler with only one Kind; configload.kindContent with only one kind; avoid double iteration

---

Reviewer checklist

• Read PR description: a summary about the changes is required
• Changelog updated
• Documentation: docs/{Reference, Cli, ...}, Docker and cli help/usage
• Pulled branch, manually tested
• Verified requirements are met
• Reviewed the code
• Reviewed the related tests

[johakoch]

errorHandlerContent's map key (the kind) is here only used for the uniqueness check. To get the actual kind kindContent.kind is used.

Another approach:

type errorHandlerContent map[string]hcl.Body

• use the map key as actual kind: pass both kind string and body hcl.Body instead of content kindContent to newErrorHandlerConfig()
• use them instead of content.kind and content.body
• the kindContent type is no longer necessary

[malud]

closed in favor of #462