Any person willfully neglecting or refusing obedience to such subpoena, or neglecting or refusing to appear and testify when subpoenaed, his fees having been paid if demanded, shall be deemed guilty of a misdemeanor, for which he shall be punished by indictment in the district court of the United States or in the district courts of the Territories exercising the jurisdiction of district courts of the United States. The punishment for such offense, upon conviction, shall be a fine of not more than $200, or imprisonment not to exceed ninety days, or both, at the discretion of the court: Provided, That if such witness has been prevented from obeying such subpoena without fault upon his part he shall not be punished under the provisions of this section.