Issue #2418929: Turn down noise in watchdog

We lower the watchdog level from WATCHDOG_ERROR to WATCHDOG_NOTICE.

It is not an error causing jobs not to be launched during maintenance
mode but the intended behavior of Ultimate Cron.

Although it is a normal behavior it is still a significant condition and
that's why we log it as WATCHDOG_NOTICE.

This will actually not result in fever entries in the watchdog, but it
will move this to a lower level and thus make it easier filter only the
more serious events.