This was reported by Stanislava Panova (https://gna.org/users/stacy). The problem was related to
the Monte Carlo simulations. The generic_fns.monte_carlo.create_data() function is now asking for
the errors associated with a certain data point, even if that data point does not exist. The
solution was to check if the relaxation data ID string is present in spin.ri_data_err before
accessing the key-value pair (in the specific API _return_error_relax_data() common method).
