SAN FRANCISCO, May 29 (Reuters) - Pacific Gas & Electric Co. (NYSE:PCG - News) has challenged an order by state regulators needed for California to launch an $11.1 billion bond issue to help recoup the cost of the state's emergency power purchases last year. The challenge, made late on Tuesday by the PG&E Corp. subsidiary, California's biggest utility, could delay the bond issue, one of the largest municipal bond sales ever, credit analysts said. While state Treasurer Phil Angelides has not set a schedule for any bond deal, he recently told investors he would likely issue a timetable sometime...