CRM-12892 - CRM/Form/validate.tpl - Clarify error about date formatting

Also: Fix loading of custom error messages

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* CRM-12892: CiviHR - Entity - Job Positions & Roles
  http://issues.civicrm.org/jira/browse/CRM-12892
CRM-12892 - CRM/Form/validate.tpl - Don't put messages in CRM.validate defaults

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* CRM-12892: CiviHR - Entity - Job Positions & Roles
  http://issues.civicrm.org/jira/browse/CRM-12892
Merge pull request #1433 from totten/master-validate-messages

CRM-12892 - CRM/Form/validate.tpl - Clarify error about date formatting