Report stdout/stderr errors gracefully

[mjambon]

Test functions should not close stdout or stderr since these are managed by testo. For example, the test function (fun () -> close_out stdout) results in an uncaught exception when or after running the test:

test: internal error, uncaught exception:
      Sys_error("Bad file descriptor")
      Raised by primitive operation at CamlinternalFormat.output_acc in file "camlinternalFormat.ml", line 1908, characters 48-63
...

I don't think we can prevent user code from accidentally closing stdout or stderr but maybe we can help the user fix their test function:

1. Catch exceptions when calling flush, close, etc.
2. Explain that there's either a bug in testo or that a specific test closed stdout or stderr.