Fix OOM handling.

First, fix LoginManager's handling of InvocationTargetException that
the underlying Throwable is logged (not the ITE, which has little
information); second that if the underlying Throwable is an Error then
rethrow it straight away (to shutdown dCache).

Second, when the unhandled Exception Handler receives a
VirtualMachineError a second time, it forces an immedate shutdown of
the JVM.  The reason is that a shutdown has been triggered with a subsequent
Error.  At this point we have no confidence that the shutdown will ever
complete, nor in which state dCache is.  Rather than risking further damage
it is better to do an emergency stop and try to recover elsewhere.