various: do not include file names directly in error messages

git grep -l 'Failed to open /'|xargs sed -r -i 's|"Failed to open (/[^ ]+): %m"|"Failed to open %s: %m", "\1"|g'
git grep -l $'Failed to open \'/'|xargs sed -r -i $'s|"Failed to open \'(/[^ ]+)\': %m"|"Failed to open %s: %m", "\\1"|g'
git grep -l "Failed to open /"|xargs sed -r -i $'s|"Failed to open (/[^ ]+), ignoring: %m"|"Failed to open %s, ignoring: %m", "\\1"|g'
+ some manual fixups.

src/shared/killall.c

@@ -235,7 +235,7 @@ static int killall(int sig, Set *pids, bool send_sighup) {
 
         r = proc_dir_open(&dir);
         if (r < 0)
-                return log_warning_errno(r, "Failed to open /proc/: %m");
+                return log_warning_errno(r, "Failed to open %s: %m", "/proc/");
 
         for (;;) {
                 _cleanup_(pidref_done) PidRef pidref = PIDREF_NULL;