NS_ERROR_NET_RESET and NS_ERROR_NET_INTERRUPT on an https URI are not evidence
of TLS version intolerance: the heuristic applies only during the handshake,
insecure fallback was disabled in bug 1479501, and genuine TLS failures reach
nssFailure2 by their own path.
Give netReset and NS_ERROR_NET_EMPTY_RESPONSE distinct error-page configs, and
test the empty-response case with an actual empty response.
Differential Revision: https://phabricator.services.mozilla.com/D323597