CRM-12371 - Don't show errors if admin refreshes post-upgrade screen.

Note: It's not possible to display the exact same messages because the session is destroyed as part of the upgrade process.

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* CRM-12371: Refreshing upgrade finish page causes errors
  http://issues.civicrm.org/jira/browse/CRM-12371
Merge pull request #561 from totten/upgrade-finish-reload

CRM-12371 - Don't show errors if admin refreshes post-upgrade screen.