Add error DSL output

[devslash-paul]

There's some times where an HTTP error, or service error will occur. In that event something should happen, currently it just prints to the stderr.

I'm imagining something such as

runHttp {
  call("http://example.com") {
    onError {
      +StdOut()
      +AppendFile()
    }
  }
}

In general, it'd be much like after except without the response. This should allow for transitive failures to be retried.

It could also allow for onErrors that would feed back into the data provider

[pault-t-canva]

This has been started - but currently there's only a single one. Also, at the moment it's also only for errors that are done when the Http request is in flight, not on before or afters

[devslash-paul]

been in master for ages.