RFR (jaxp): 8015016: Improve JAXP 1.5 error message

[huizhe wang]

This is a quick fix to improve the error message so that it's clearly 
differentiated from those reported by the Security Manager.

The change is identical for both JDK7 and 8. I will backport the fix to 
7u40 if approved.

webrev:
http://cr.openjdk.java.net/~joehw/jdk8/8015016/webrev/

Thanks,
Joe