kill: report error if cannot kill process

Shell kill would report a problem if you tried to kill a process
while procps kill was silent. This meant it looked like kill worked
when it actually failed.

References:
 https://bugs.debian.org/733172

skill.c

@@ -500,6 +500,7 @@ static void __attribute__ ((__noreturn__))
 		pid = strtol_or_err(argv[i], _("failed to parse argument"));
 		if (!kill((pid_t) pid, signo))
 			continue;
+        error(0, errno, "(%d)", pid);
 		exitvalue = EXIT_FAILURE;
 		continue;
 	}