Catch `ValueError` when trying to raise the max open files setting. Fixes #1991.

Under some occasions, the current default of 100000 max open files setting is too high. Properly catch that exception and warn the users about it.
See https://github.com/saltstack/salt/issues/1991#issuecomment-13025595
Merge pull request #3570 from s0undt3ch/develop

Catch `ValueError` when trying to raise the max open files setting. Refs #1991