Amid growing questions over the future of insurance exchanges, the head of California's marketplace said the nation's largest health insurer should take responsibility for nearly $1 billion in losses and stop blaming the federal health law. In a blistering critique, Covered California's executive director, Peter Lee, said UnitedHealth Group Inc. made a series of blunders on rates and networks that led to a $475 million loss last year on individual policies across the country.