Unexpected error message in get-outputs-amounts

[onvej-sl]

  {
    "Name": "",
    "Request": {
      "InputSize": 58,
      "OutputSize": 43,
      "AvailableVsize": 394,
      "MiningFeeRate": 129000,
      "AllowedOutputAmounts": {
        "Min": 5000,
        "Max": 4300000000000
      },
      "InternalAmounts": [
        4942,
        4942,
        4942
      ],
      "ExternalAmounts": [
      ]
    },
    "ExpectedStatusCode": 500,
    "ExpectedResponse": {
      "Description" : "Value cannot be null. (Parameter 'left')."
    }
  }

[onvej-sl]

We should fix the behaviour of get-outputs-amounts so that the test returns Not enough coins registered to participate in the coinjoin. instead of Value cannot be null. (Parameter 'left')..