Warn when we fail to trace and SELinux boolean deny_ptrace is in effect
diff --git a/proc.c b/proc.c
index 106b6a0..ded0c95 100644
--- a/proc.c
+++ b/proc.c
@@ -94,6 +94,7 @@
 	if (open_one_pid(pid)) {
 		fprintf(stderr, "Cannot attach to pid %u: %s\n",
 			pid, strerror(errno));
+		trace_fail_warning(pid);
 		return;
 	}