RFR (S) 8153413: Exceptions::_throw always logs exceptions, penalizing performance

[coleen.phillimore at oracle.com]

Summary: construct exception string into Event message directly.

This is somewhat of a minimal change that has a small benefit to 
performance (see bug for more details).  It's been merged with 
Yasumasa's change.

Tested with hs-tier1-2 and performance tests.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8153413.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8153413

Thanks,
Coleen