RFR: JDK-8188856: Incorrect file path in an exception message when .java_pid is not accessible on Unix

[Gary Adams]

Here's a simple fix to correct the error message when the java_pid socket
is not found. The code previously reported the attach_pid file name
rather than the socket file name that was not found.

  Issue: https://bugs.openjdk.java.net/browse/JDK-8188856
  Webrev: http://cr.openjdk.java.net/~gadams/8188856/webrev.00/