http.server: Move error message formatting to handler class

Like before this patch moves more functionality from the actual server
class into a separate handler class. At the same time the function is
changed to return both content-type and body instead of relying on a
class attribute.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>