JENKINS-2673, JENKINS-1406 Fix inappropriate cron warnings

As the other commenters on https://issues.jenkins-ci.org/browse/JENKINS-2673 , 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 have been unable to register an account on the Jenkins JIRA (something about LDAP errors), so please could whoever merges this comment on both these two issues to note this change, and close 2673?

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