show details for missing polkit failure

a common error is that openvpn cannot be launched because there's no polkit agent running. after I merge some fixes, the UI will mark the error state as "failed", but it gives no verbose reason.

this probably will work on the second time (there's a timing issue with our attempt to launch the polkit agent), but I want to make sure that we are displaying something useful for the user in the case of unrecoverable errors.

this is the json that the ui will be receiving:

{
  "result": {
    "status": "failed",
    "domain": "demo.bitmask.net",
    "childrenStatus": {
      "firewall": {
        "status": "off",
        "error": null
      },
      "vpn": {
        "status": "failed",
        "error": "No polkit authentication agent available. Please run one."
      }
    },
    "error": "No polkit authentication agent available. Please run one."
  },
  "error": "No polkit authentication agent available. Please run one."
}