SDE connection failed message non descriptive

[mchapman]

When trying to connect to sde in OGRSDEDataSource::Open() using an IP address the api returns an empty or non descriptive error making it appear as though a critical error occurred.  The solution is to pass a server UNC or friendly name.  This wasn't clear to me and I spent a little while trying to figure it out, so I thought by adding a check for an ip address or localhost it would be helpful to new users of the driver.  

Is that ok with you Howard/Frank?