BackendCardDAV: invalid return type "false" in ChangesSink()

If the CardDAV server is suddenly not available, ChangesSink() will stop looping through addressbooks and return a boolean value, where an array is expected. (backend/carddav/carddav.php, line: 261)

As an error message is logged anyway and server availability may only be affected temporarily, I think it is better to go on with the next addressbook using continue.
No change will be detected this time, but most likely in the next execution of ChangesSink().