When should evidentiary errors in a federal criminal trial be evaluated for their cumulative impact on the fundamental fairness of the trial?

Question Presented When, on direct appeal from a conviction in a federal criminal trial, an appellate court identifies multiple evidentiary errors, should each of those errors be evaluated for their impact on the fundamental fairness of the trial seriatim in isolation or in an assessment of their cumulative impact, and should the standard of review place the burden of proof on the government to show that the errors did not impact the verdict beyond a reasonable doubt as required by Chapman v. California, 386 U.S. 18 (1967), and Sullivan v. Louisiana, 508 U.S. 275 (1993)? 1