carddav_discovery just return false to the client instead of a meaningful error message

[tfl]

The other day I spend a few hours to get the plugin working. I used wrong login credentials in my carddav configuration and I was sure I was using the right one - which I was not ;)

The plugin did not show me that the CardDAV server returned 401. Looking through the code I see that on problems or errors it just returns $false - without even logging to the logfile. And how about returning not just $false on errors but instead return a nice return object containing a status code, data and the error message if there where errors. This way the plugin should be able to show the error to the user.

[mstilkerich]

Normally, RCMCardDAV would show a 401 error when the password is wrong, e.g. when connecting to an addressbook already added after a password change at the server side not taken over to RCMCardDAV. The addressbook discovery process is a bit difficult though. There is different discovery mechanisms (via DNS SRV, /well-known/ URI) that may or may not be set up by the admin of the CardDAV service. Thus the discovery process includes probing of locations where a failure does not necessarily mean that the data provided by the user is faulty, but could also mean that the probed location is not provided by the service. When the discovery eventually fails, the last occurred error is not necessarily the one we would like to report to the user, because an earlier probed location that failed only because of failed credentials might have been the correct discovery path, not the finally probed one.

To provide a more detailed failure reason, I must check more in depth for which of the failures we can assume an error on our side to pick the error to report upwards. This has to happen in the carddavclient lib and will likely need backwards incompatible API change. Thus I'm postponing this issue for the 5.0 release.