drop errors.php and simplify error handling

api/index.php

backend.php

classes/api.php

classes/feeds.php

classes/handler/public.php

classes/pluginhandler.php

classes/pref/feeds.php

classes/pref/labels.php

classes/pref/prefs.php

classes/pref/users.php

classes/rpc.php

errors.php

include/functions.php

include/login_form.php

index.php

js/App.js

js/common.js

prefs.php

public.php