🍒polkit: don't ignore fcntl errors

treat them as internal errors and warn about them

previously we'd ingore the errors and use an invalid fd, this would later manifest in infinite write loops on the invalid file descriptor (i.e. all writes fail because the fd is garbage leading to an effective inifinite loop while trying to write segments in coredumpexcavator.cpp)

CCBUG: 487727 (cherry picked from commit 245cc609)