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

NEWS

@@ -5,6 +5,7 @@ procps-ng-NEXT
   * ps: Fallback to attr/current for context Debian #786956
   * tests: Conditionally add prctl Debian #816237
   * pidof: check cmd if space in argv0. GitLab #4
+  * kill: report error if cannot kill process #733172
 
 procps-ng-3.3.11
 ----------------

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;
 	}