MINOR: systemd: report it when execve() fails

It's important to know that a signal sent to the wrapper had no effect
because something failed during execve(). Ideally more info (strerror)
should be reported. It would be nice to backport this to 1.6 and 1.5.
(cherry picked from commit a785269b4e09114224062081f02b443b6008c524)
(cherry picked from commit 3bc0a399d2875e9e0627a70906c1403eef60ec60)
diff --git a/src/haproxy-systemd-wrapper.c b/src/haproxy-systemd-wrapper.c
index 15c48ca..f4fcab1 100644
--- a/src/haproxy-systemd-wrapper.c
+++ b/src/haproxy-systemd-wrapper.c
@@ -107,8 +107,12 @@
 		fprintf(stderr, "\n");
 
 		execv(argv[0], argv);
+		fprintf(stderr, SD_NOTICE "haproxy-systemd-wrapper: execv(%s) failed, please try again later.\n", argv[0]);
 		exit(1);
 	}
+	else if (pid == -1) {
+		fprintf(stderr, SD_NOTICE "haproxy-systemd-wrapper: failed to fork(), please try again later.\n");
+	}
 }
 
 static int read_pids(char ***pid_strv)
@@ -180,6 +184,7 @@
 	execv(wrapper_argv[0], wrapper_argv);
 	/* failed, let's reinstall the signal handler and continue */
 	setup_signal_handler();
+	fprintf(stderr, SD_NOTICE "haproxy-systemd-wrapper: re-exec(%s) failed.\n", wrapper_argv[0]);
 }
 
 /* handles SIGTERM and SIGINT only */