PHILADELPHIA — A jury convicted Msgr. William J. Lynn of child endangerment Friday, finding that as the Archdiocese of Philadelphia secretary for clergy, he ignored credible warning signs about a priest who later sexually assaulted a 10-year-old altar boy. The verdict, after a three-month trial, marked the first time since the clergy sex-abuse scandal erupted a decade ago that a Catholic Church supervisor has been found criminally liable for child-sex crimes by a priest. Common Pleas Court Judge M. Teresa Sarmina immediately revoked Lynn’s bail, and deputy sheriffs escorted the white-haired monsignor to a holding cell. Lynn faces up to seven years in prison, and prosecutors vowed to seek a term near the maximum.