Summary: Currently HTML renders as a blank response panel which is not helpful for seeing erros. Adding a formatter for `text/html` so we can see error responses, or other HTML responses, correctly. Note, this adds the xml beautifier package from npm. Reviewed By: danielbuechele Differential Revision: D12883274 fbshipit-source-id: c49b60984f10bba471b5bbb8aabcb385a171d0d6