“ Where state procedural snarls or obstacles preclude an effective state remedy against unconstitutional convictions, federal courts have no other choice but to grant relief in the collateral proceeding. ”

“ But the situation is different in federal proceedings, over which both the Courts of Appeals and this Court (McNabb v. United States, 318 U.S. 332, 63 S.Ct. 608, 87 L.Ed. 819) have broad powers of supervision. It is more appropriate, whenever possible, to correct errors reachable by the appeal rather than remit the parties to a new collateral proceeding. ”