MBeanInvocationHandler should throw the cause of InvocationTargetException, not ITE itself

KARAF-7234 fixed KARAF-6654 by throwing the exception from MBean invocation instead of returning null.
However, the exception thrown by org.apache.karaf.management.internal.MBeanInvocationHandler#invoke() should be the cause of java.lang.reflect.InvocationTargetException, not this exception itself.

Now, jconsole gets UndeclaredThrowableException instead of the actual exception.