Improve error messaging for invalid matrix entry

[SimonBradley1993]

I'm currently investigating an issue with some requests being sent to Vroom coming back with Invalid matrix entry. I'd like to improve the error message here to include the matrix entry that caused this issue. The check is being performed here :

      if (!mi[j].IsUint()) {
        throw InputException("Invalid matrix entry.");
      }
      matrix[i][j] = mi[j].GetUint();
    }
  }

And I'd like to change this to :

      if (!mi[j].IsUint()) {
        throw InputException("Invalid matrix entry {}.", mi[j]);
      }
      matrix[i][j] = mi[j].GetUint();
    }
  }

[jcoupey]

Did you try building with this kind of change? mi[j] could be really any other type than uint, so I suspect you may not be able to turn a generic rapidjson object into a string that easily.