Fix stressHistogram and ErrorIntegral tutorial for using TF1::IntegralError...

Fix stressHistogram and ErrorIntegral tutorial for using TF1::IntegralError when running in MT (IsImplicitMTEnabled).
In this case the global TVirtualFitter object is not existing. Need to pass covariance matrix from TFitResult.

Update error message when using TF1::IntegralError when running in MT .

Update also documentation to indicate this case