do not ignore write error for any output size

do not ignore write error for any output size

The previous change was incomplete.
While it makes emacs --batch detect the vast majority of stdout
write failures, errors were still ignored whenever the output size is
k * (BUFSIZ+1) - 4.  E.g., on a system with BUFSIZ of 4096,
  $ emacs --batch --eval '(print (format "%4093d" 0))' > /dev/full \
      && echo FAIL: ignored write error
  FAIL: ignored write error
  $ emacs --batch --eval '(print (format "%20481d" 0))' > /dev/full \
      && echo FAIL: ignored write error
  FAIL: ignored write error
* emacs.c (Fkill_emacs): Also test ferror.  (Bug#9574)