src/Tweeper.php: harmonize error messages

Since the Tweeper class is supposed to be used as a library don't let
any error be fatal and convert all current uses of E_USER_ERROR into
E_USER_WARNING.

Also convert the few instances of E_USER_NOTICE into E_USER_WARNING.

Finally, stop using error_log as well in favour of trigger_error which
provides more context in the produced message.