Holding that "instructions and counsel's arguments thereon viewed as a whole, sufficiently informed the penalty phase jury it could consider a mental condition of the defendant which, though not characterized as extreme, would potentially mitigate the circumstances of the offense"
Holding that a court can determine if it possesses the requisite discretion to consider harmless error by considering "the length and complexity of the record, whether harmlessness of the error or errors is certain or debatable, and whether the reversal will result in protracted, costly, and ultimately futile proceedings in the district court"