RFR 8202708, Add a check of opening stream for not-existing UNC url

[Felix Yang]

Hi all,

    please review a change to add additional checking for opening stream 
with not-existing UNC url.

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8202708

Webrev:

     http://cr.openjdk.java.net/~xiaofeya/8202708/webrev.00/

Thanks,

Felix