I think I got it.
By just removing the first three breaks, I triggered the unknown error case and could see the result.
So to answer my own questions:
Was it predefined anywhere else? No, and also I understand that it didn't need to be.
What will it return? error.UNKNOWN_ERROR simply returns 'undefined' and it's not a problem at all.
How can I cause an (intentional) unknown error to trigger this case? I didn't need to cause a specific run-time unknown error to trigger the last case ,
I just removed the first three breaks and the last case was triggered, so I could see that error.UNKNOWN_ERROR returns 'undefined' and realize that it is just fine.
"An unknown error occurred" message is printed on the page as it should be.
Thanks!