system: dont let any errors interfere with the shutdown process and just

I think there is a bug in the shutdown process. Just start Factor then, with nothing on the stack, type:
```factor
IN: scratchpad exit
```
What now happens is that the shutdown hooks run, then the `(exit)` word throws a datastack underflow error which aborts the shutdown process. But the shutdown hooks have already run so Factor can be in a bad state and crash or behave weirdly. 

There's no great way to handle the situation. So what I think is best is just to ignore any errors and exit with an error because you can't recover. That matches the way Python and other VM languages handles shutdown errors. Unfortunately you can't even pretty print the error itself because prettyprint is in basis and the system vocab is in core. 

Oops, merge commit I think. Fixed.