tty-ask-password-agent: don't warn if we cannot disable wall messages on the calling tty

For example, due to perm issues.

THis simply downgrades the message about it, since this is purely
cosmetical anyway.

Fixes #1543.

src/tty-ask-password-agent/tty-ask-password-agent.c

@@ -381,7 +381,7 @@ static int wall_tty_block(void) {
 
         fd = open(p, O_RDONLY|O_CLOEXEC|O_NONBLOCK|O_NOCTTY);
         if (fd < 0)
-                return log_error_errno(errno, "Failed to open %s: %m", p);
+                return log_debug_errno(errno, "Failed to open %s: %m", p);
 
         return fd;
 }