Fix check not updated when fetch package errors

When a package was removed from AUR, the check code didn't update the last_check
value and the loop code was always triggered.

[DEBUG] Next check is planned in -778781s, waiting for 1s
...
[DEBUG] Next check is planned in -778781s, waiting for 1s
...
[DEBUG] Next check is planned in -778781s, waiting for 1s
...
[DEBUG] Next check is planned in -778781s, waiting for 1s