JENKINS-2673, JENKINS-1406 Fix inappropriate cron warnings

As the other commenters on https://issues.jenkins-ci.org/browse/JENKINS-2673 found, I was surprised to see a slightly confusing warning from Jenkins when I set my update schedule to "`* * * * *`".

This patch should:
 * Not issue the minutes / hours warning in the "`* * * * *`" case, as it seems inappropriate
 * Improve the warning in the cases where it is appropriate (see JENKINS-1406 for motivating examples)

I wasn't able to run the unit tests: would it be possible for whoever picks this up to run the tests before merging?

Thanks,


Rich