JShell: three small proposal/feedback

[Brian Goetz]

>>
>> Unfortunately, I did not think the default editor uses swing, so I got
>> HeadlessException when I used /edit on a headless server, then
>> my working log has gone. So I want that JShell warn instead of
>> suddenly quit, as [1].
>
> That would be good.

I might extend this to "catch (RuntimeException e)" -- if there's any 
problem loading the editor, the show should go on.