If terminal is too small, error thrown
Not really a bug, but I get something like:
```
/home/user/e.nim(71)     e
/home/user/e.nim(66)     main
/home/user/e.nim(44)     updateScreen
/usr/lib/nim/system/fatal.nim(49) sysFatal
Error: unhandled exception: value out of range: -1 notin 0 .. 9223372036854775807 [RangeDefect]
```
whenever I resize the screen to something too small. this makes sense, and I know I can error handle it, but I'm curious where it occurs so that I could avoid blanket error handling all of it? Maybe Illwill could throw a better error