Which of the following combinations is the correct answer?
Even if your FEA software reports no error, the solution may not be correct. No error means only that the model is correct from the “solver point of view.” The solver will solve the most incorrect model as long as it does not run into
numerical problems.
You do not really need any error estimation, and the FEA is always accurate enough.
You should always make very fine mesh, so you do not have to worry about error.