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:
 commit 07642b8ea6
 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
 ----------------

kill.c

@@ -132,6 +132,7 @@ int main(int argc, char **argv)
         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;
     }