dumps errors as valid JSON

  when status code is != 200
  when response has invalid JSON

  the output file will be something like:
```json
{
   "response" : {
      "value"   : "FAIL",
      "error"   : "Timeout connecting to host",
      "content" : "503 Bad Gateway"
   }
}
```

or a more realistic example 
```json
{
   "response" : {
      "value" : "401 Unauthorized",
      "content" : "<html>\r\n<head><title>401 Authorization Required</title></head>\r\n<body bgcolor=\"white\">\r\n<center><h1>401 Authorization Required</h1></center>\r\n<hr><center>ngx_openresty/1.2.7.6</center>\r\n</body>\r\n</html>\r\n",
      "error" : "not a 200"
   }
}
```