MIAMI — A political crisis engulfed Puerto Rico over the weekend, prompting the departure of two senior members of the government and threatening Gov. Ricardo A. Rosselló, who found himself increasingly isolated in office and no longer supported by leaders of his own party. Mr. Rosselló’s administration was rocked by the publication of a trove of derisive messages sent by the governor and some of his cabinet members and top aides in a private chat on the messaging app Telegram. The messages mocked political foes and allies alike, often with profanity. On Saturday, the governor tried to contain the fallout...