A federal judge heatedly scolded prosecutors in the criminal trial of W. R. Grace Monday for their mishandling of the case. At a hearing, Judge Donald W. Molloy questioned the prosecutors’ judgment, ethics and tactics. Attorneys for Grace and five of its executives, accused of knowingly contaminating the small town of Libby, Mont., have argued that prosecutors withheld evidence and put on the stand a witness who may have perjured himself.