Using the grid search meta-estimator with the "mean_square_error", the "mean_absolute_error", the "median_absolute_error" or the "log_loss" as scoring parameters leads to the negation of those metrics. This is confusing especially for new users.

I suggest that we prefix those strings by "neg-" or "negative_". This would make clear from the start that the score is obtained from the negation of the loss / error.