Fix export_marks() error handling.

Fix export_marks() error handling.

- Don't leak one FILE * on error per export_marks() call. Found with
  cppcheck and reported by Martin Ettl.

- Abort the potentially long for(;idnums.size;) loop on write errors.

- Record error if fprintf() fails for reasons not required to set the
  stream error indicator, such as ENOMEM.

- Add a trailing full-stop to error message when fopen() fails.

Signed-off-by: Matthias Andree <matthias.andree@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>