"In order to grant a writ of error coram nobis there must be shown the existence of an error of fact which was unknown at the time of trial and which is of such a nature that knowledge of its existence at the time of trial would have prevented the entry of judgment.

—to other arguments advanced by respondent and amicus curiae, we perceive no tenable theory upon which to predicate a conclusion that a judicial refusal to furnish, on request, appointed counsel to an indigent state prisoner prosecuting, at the hearing or first appellate level, a postconviction action which presents an issue that is urged in good faith, is not frivolous or …

The writ of coram nobis is traditionally a discretionary writ granted for consideration of a fact which may change the decision and which wvas not known at the trial.