bug#50511: 27.2; url-http-handle-authentication should not raise error

[Jonas Bernoulli]

Since [1: 64b469f6ae] url-http-handle-authentication raises an error
when it detects that we already tried to make an authenticated request.

1: 2019-07-26 64b469f6ae8173116ec948ac43cd44efe4b5a221
   Don't infloop in url.el when sending invalid basic auth

Instead of signaling an error using `error', it should simply return t
to indicate that no further requests should be made and the response
data (in this case error data) should be passed to the handler, i.e. to
indicate that the request "successfully failed".  Then the error handler
can inspect the response and decide how to react.

This is how it is done for all other https response codes, including all
other error codes.  This approach also solves the infloop issue but also
makes it possible to use an error handler as intended.

     Jonas