package main

import
(
  "encoding/json"
  "fmt"
)

type DeftyError {
  Error SubError
}

type SubError struct {
  Message string
}

func(e *DeftyError) Error() string {
  return e.Error.Message
}

type Thing struct {
}

func(t *Thing) MethodReturningError() error {
  if true {
    return &DeftyError{
      Error: SubError{
        Message: "£400 please",
      }
    }
  } else {
    return fmt.Errorf("Some other type of error")
  }
}
func main() {
  mystruct := &Thing{}
  err := mystruct.MethodReturningError()
  if err != nil {
    concreteStruct, ok := err.(*DeftyError)
    if ok {
      // it's our error! json encode it
      output, err := json.Marshal(concreteStruct)
      if err != nil {
        panic(err)
      }
    } else {
      // it's another type of error which we don't know how to marshal/don't want to marshal.
      panic(err)
    }
  }
}