Implement failure only transforms

[Tratcher]

Fixes #1256

Response transforms can be conditionally run on success or failure, as indicated by the response status code being greater or less than 400. Note if there is no proxy response then HttpForwarder set the response status code to 50x before running transforms.

[MihaZupan]

Is there an example use case we could add to the transforms doc?
Afaik we don't have a StatusCode transform for something like #1256 (comment)

[Tratcher]

The example wouldn't look any different than the ones already given in the docs.

{
  "ResponseHeader": "HeaderName",
  "Append": "value",
  "When": "Failure"
}