Return better error message from graphql when transactions are missing (not up to date)

[Matthew Darwin]

Current error message could be improved:

{
  "errors": [
    {
      "message": "graphql: panic occurred: runtime error: invalid memory address or nil pointer dereference",
      "path": [
        "searchTransactionsForward"
      ]
    }
  ],
  "data": null
}