Changed the code so that it stops when more than 20% of the models failed to produce the covar matrix: non invertible hessian

Basically added a few lines of code so that the model takes the average without the NAs (these are for the non invertible hessian cases).
If we have <=20% of NAs, then the code proceeds to the next phase 
BUT if we have >20% of NAs in our simulation it stops there and produces an error message indicating which iterations failed to produce invertible hessian.