e_cal_backend_get_timezone_finish: Do not panic when the timezone cannot be found

When the specific timezone could not be found, the function threw a runtime
warning, and did not set an error, which further in the call made fail also
data_cal_complete_get_timezone_cb(), which has a check for "either have
a timezone object or an error", which finally prevented a correct finish
of the D-Bus call, thus the client side which asked for an unknown timezone
was left stuck, waiting for a response from the server which was not
interested in the call anymore.

This behaviour could lead to a UI freeze, as had been reported at:
https://bugzilla.redhat.com/show_bug.cgi?id=1075461