Corrected error message when doing warm start with n_new_estimators < 0.
Removed useless list cast and used extend instead of  += for more clarity.
Renamed n_new_estimators to n_more_estimators
Moved random_state alteration right before the generation of `trees`.
Changed error raising condition as fitting with n_more_estimators==0 raises a division by 0 in _partition_estimators.
Added test for the proper growth of the forest with warm start.
Also tried to make the test clearer (use of set_params, variable names).