The extent of the nation’s housing crisis was tangible Thursday morning. It stretched the length of one side of Palm Beach County’s Convention Center as hundreds of homeowners, wrapped in blankets and travel-weary after flying from as far away as California, waited for the salvation of a mortgage modification. Welcome to the “Save-a-Thon” — five days of around-the-clock loan woe turnarounds sponsored by the Boston-based Neighborhood Assistance Corporation of America, or NACA.