RFR: 6696970: JConsole becomes unusable if a plugin throws an exception

[Erik Gahlin]

Please review this change.

An exception from JConsole plug-in should not affect other tabs.
http://monaco.us.oracle.com/detail.jsf?cr=6696970

Added a plug-in proxy that handles RE. If debug mode is enabled 
exceptions are written to System.err. Tested by adding faked exceptions 
to the JTop plug-in.

Code is targeted for jdk7.

Webrev:
http://cr.openjdk.java.net/~egahlin/6696970

A sponsor is needed.

Thanks
Erik