CRM-13790 - Smart group cache timeout description is wrong

http://issues.civicrm.org/jira/browse/CRM-13790
CRM-13790 - improve error message

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* CRM-13790: Smart group cache timeout description is wrong
  http://issues.civicrm.org/jira/browse/CRM-13790
Merge pull request #2020 from dlobo/CRM-13790

CRM-13790 - Smart group cache timeout description is wrong