util-scl/oo-mco: Added error message when scl mco does not exist.

Bug 1131139
Bugzilla link https://bugzilla.redhat.com/show_bug.cgi?id=1131139
oo-mco is installed on nodes due to rpm configuration, however the scl mco executable is not. Added error message to avoid confusion for newer administrators.
Merge pull request #5719 from tiwillia/bz1131139

Merged by openshift-bot
Automatic commit of package [rubygem-openshift-origin-node] release [1.30.5-1].
Automatic commit of package [openshift-origin-cartridge-jbosseap] release [2.20.3-1].
Automatic commit of package [openshift-origin-logshifter] release [1.7.3-1].
Automatic commit of package [openshift-origin-util-scl] release [1.18.2-1].
Automatic commit of package [openshift-origin-node-util] release [1.29.3-1].