RFR 8157917: JShell: shutdown could cause remote JDWP errors to be visible to users

[Robert Field]

Please review

Bugs:

     8157917: JShell: shutdown could cause remote JDWP errors to be 
visible to users
     https://bugs.openjdk.java.net/browse/JDK-8157917

     8157918: JShell tests: StartOptionTest displays insufficient 
information to diagnose failures
     https://bugs.openjdk.java.net/browse/JDK-8157918

Webrev:

     http://cr.openjdk.java.net/~rfield/8157917v0.webrev/

Thanks,
Robert