RFR: 8074812 More specific error message when the .java_pid well-known file is not secure

[Staffan Larsen]

During attach, if the .java_pid file is not secure we currently say "well-known file is not secure". This can be enhanced to say _why_ the file is not considered secure.

bug: https://bugs.openjdk.java.net/browse/JDK-8074812
webrev: http://cr.openjdk.java.net/~sla/8074812/webrev.00/

Note that it is safe to call JNU_ReleaseStringPlatformChars after the calls to JNU_ThrowIOException since the latter don’t actually change the control flow.

Thanks,
/Staffan