bug(sync): sync ignores errors handling when the file is /proc/self/mem and /dev/full

Hi, uutils mainteners

very weird bug that should not happen but does, when using /proc/self/mem the uutil sync ignore errors, but gnu one errors out, and the uutils one exit with a misleading 0 status exit code as the testing confirms

relunsec@relunsec:~/software/coreutils/target/debug$ gnusync /proc/self/mem
gnusync: error syncing '/proc/self/mem': Invalid argument
relunsec@relunsec:~/software/coreutils/target/debug$ ./sync /proc/self/mem
relunsec@relunsec:~/software/coreutils/target/debug$ echo $?
0
relunsec@relunsec:~/software/coreutils/target/debug$ 
relunsec@relunsec:~/software/coreutils/target/debug$ ./sync /dev/full
relunsec@relunsec:~/software/coreutils/target/debug$ gnusync /dev/full
gnusync: error syncing '/dev/full': Invalid argument
relunsec@relunsec:~/software/coreutils/target/debug$ 

as you can the uutils status code is 0 and ignores handling errors

Start with the sync command's error handling and reproduce the reported cases using /proc/self/mem and /dev/full, comparing its output and status with GNU sync. Done means these inputs report the sync error and exit with a nonzero status instead of silently returning 0.