Avoid 'Failed to start monitoring' HttpException when resource is in a second-level directory

Ref
* http://forums.asp.net/t/883275.aspx
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http://social.msdn.microsoft.com/Forums/windowsazure/en-US/443fd34d-c88f-4375-87ff-2008304bb46d/virtual-path-provider-giving-directory-esitesroot0xxx-does-not-exist-failed-to-start