Add error checking to confirm bug

There seems to be an error in the varnish jail design, which makes the
mgt process uable to kill the child process. To confirm this, add some
error checking to the relevant code.

Related to: varnishcache#2010

bin/varnishd/mgt/mgt_child.c

@@ -34,6 +34,7 @@
 #include <sys/types.h>
 #include <sys/wait.h>
 
+#include <errno.h>
 #include <fcntl.h>
 #include <poll.h>
 #include <signal.h>
@@ -583,17 +584,22 @@ mgt_reap_child(void)
 void
 MGT_Child_Cli_Fail(void)
 {
+	int i;
 
 	if (child_state != CH_RUNNING)
 		return;
 	if (child_pid < 0)
 		return;
-	MGT_complain(C_ERR, "Child (%jd) not responding to CLI, killing it.",
-	    (intmax_t)child_pid);
 	if (MGT_FEATURE(FEATURE_NO_COREDUMP))
-		(void)kill(child_pid, SIGKILL);
+		i = kill(child_pid, SIGKILL);
+	else
+		i = kill(child_pid, SIGQUIT);
+	if (i == 0)
+		MGT_complain(C_ERR, "Child (%jd) not responding to CLI,"
+		    " killing it.", (intmax_t)child_pid);
 	else
-		(void)kill(child_pid, SIGQUIT);
+		MGT_complain(C_ERR, "Failed to kill child with PID %jd: %s",
+		    (intmax_t)child_pid, strerror(errno));
 }
 
 /*=====================================================================