insights: UpdateInsightsDashboard does not check to see if dashboard exists

Created by: CristinaBirkel

Repro:

1. Call UpdateInsightsDashboard on a dashboard that does not exist.

Expected: dashboard not found Actual: got nil for non-null

There are multiple ways to fix this, but it seems to be because the validation returns hasPermission=true when the dashboard doesn't exist.

Example query:

mutation updatedb {
  updateInsightsDashboard(id:"ZGFzaGJvYXJkOnsiSWRUeXBlIjoiY3VzdG9tIiwiQXJnIjo4NTY4Njg2OH0=", input:{}) {
    dashboard {
      id
    }
  }
}

{
  "errors": [
    {
      "message": "graphql: got nil for non-null \"InsightsDashboardPayload\"",
      "path": [
        "updateInsightsDashboard"
      ]
    }
  ],
  "data": null
}