Merge pull request #1659 from edx/derf/mongo_version_check_should_give_better_errors_on_failure

mongo version check should give better errors on failure

playbooks/roles/mongo/tasks/main.yml

@@ -5,7 +5,7 @@
   register: mongodb_needs_upgrade
 
 - name: verify 2.4 not installed
-  fail: msg="MongoDB 2.4 is currently installed. Check http://docs.mongodb.org/manual/release-notes/2.6-upgrade/#upgrade-considerations - automated upgrade to follow"
+  fail: msg="MongoDB 2.4 is currently installed. If on a stand alone host (devstack), apt-get remove mongodb-10gen and re-run ansible. if on a cluster, read http://docs.mongodb.org/manual/release-notes/2.6-upgrade/#upgrade-considerations and upgrade to 2.6."
   when: mongodb_needs_upgrade.stat.exists