Create full Exception hierarchy in Java types

It would be much nicer in JRuby if all of the core Ruby exceptions were actually represented as Java exceptions. For example. ConcurrencyError can get out to Java code, but it always looks like RaiseException and has to be unwrapped. We would probably need the following for this to work:

• Real Java classes for all core exception types (RubyObject hierarchy)
• Real Java exceptions for all core exception types (RaiseException hierarchy)
• A protocol by which the Ruby exception type knows how to construct appropriate Java exception type

This last point is important; we should be able to construct the proper type of Java exception given only a Ruby exception type, so they stay consistent. The two types should largely be indivisible.

I started an attempt at this for 1.6, but it was too much code to introduce a week before RC.