CRM-14092 - CRM_Utils_Check_Security::isBrowseable - Fix warning

We don't care if there's an HTTP 404 when checking for browsability --
because 404 is another of saying "not browseable!"
Merge pull request #2483 from totten/4.4-security-warning

CRM-14092 - CRM_Utils_Check_Security::isBrowseable - Fix warning