Try creating a backup in a directory which does not exist. You will get the error:

An exception occurred backing up: com.atlassian.core.AtlassianCoreException: Error in action: <a href='mailto:com.atlassian.jira.action.admin.DataExport@1641c0'>com.atlassian.jira.action.admin.DataExport@1641c0</a>, result: error<br/> admin.errors.dataexport.error.exporting.data

The 'admin.errors.dataexport.error.exporting.data' property exists but isn't being used for some reason.