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

[Gary Adams]

Looking into some minor bugs in the attach area for the next release.
  https://bugs.openjdk.java.net/browse/JDK-8188856

Seems like a minor change to fix this error message that mentions the 
java_pid socket file,
but then outputs the attach_pid file when the socket file is not created .

What is the general preference for this type fix?
   - remove the reference to the "socket file" in the error message
   - construct the filename for the socket that was not found