You can find the full error for a refresh failure within the backgrounder folder within the Tableau Server logs.
For details on gathering Tableau Server logs, please visit the following KB article: http://kb.tableausoftware.com/articles/knowledgebase/creating-tableau-server-log-files
However, there is an easier way to find the full error. If you would log in to Tableau Server as the System Administrator, Select the Admin Tab, and right under the Tableau Server status graphic there is a section for Administrative Views. Select Background Task for Extracts, when the view loads there should be a view for all the background tasks currently scheduled on Tableau Server. Locate the one for the refresh and, if there is a Red "X" hover over the "X" to display an error message.