WASHINGTON — About 800,000 taxpayers who enrolled in insurance policies through HealthCare.gov received erroneous tax information from the government, and were urged on Friday to hold off on filing tax returns until the error could be corrected. ... Democratic lawmakers pressed hard for the Obama administration to open a “special enrollment period” around tax filing season for Americans who only then realize they face penalties for failing to purchase health insurance last year. Andrew M. Slavitt, the No. 2 official at the federal Centers for Medicare and Medicaid Services, said the period would last from March 15 to April 30.