RFR 8153716: JShell tool: should warn when failed to launch editor

[KUBOTA Yuji]

I filed the proposal which reported at below:
http://mail.openjdk.java.net/pipermail/kulla-dev/2016-April/001305.html

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8153716

Webrev:
  http://cr.openjdk.java.net/~ykubota/8153716/webrev.00/

I'm an author, not committer, so I need sponsor.
Please review it.

Thanks,
Yuji