on error, should flush stdout
in both cases of https://github.com/AndreiRegiani/INim/issues/29, there's a flush missing:
the result of 1st echo isn't shown

whereas `nim` itself does flush:
```
nim c -o:app -r bugs/t59_array_index.nim
...
@["a", "b", "c"] # <= HERE
/Users/timothee/git_clone/nim/timn/bugs/t59_array_index.nim(3) t59_array_index
/Users/timothee/git_clone/nim/Nim/lib/system.nim(3597) []
/Users/timothee/git_clone/nim/Nim/lib/system.nim(2807) sysFatal
Error: unhandled exception: index out of bounds [IndexError]
Error: execution of an external program failed: './app '
```
