execute: restore ability that SetCredential= can act as fallback for LoadCredential=

If SetCredential= and LoadCredentials= are combined for the same
credential name, then the former shall act as fallback for the latter in
case the source file does not exist. That's documented, but didn't work.
Let's fix that.

src/core/execute.c

@@ -2792,8 +2792,10 @@ static int acquire_credentials(
 
                 if (path_is_absolute(lc->path)) {
                         sub_fd = open(lc->path, O_DIRECTORY|O_CLOEXEC|O_RDONLY);
-                        if (sub_fd < 0 && errno != ENOTDIR)
-                                return -errno;
+                        if (sub_fd < 0 && !IN_SET(errno,
+                                                  ENOTDIR,  /* Not a directory */
+                                                  ENOENT))  /* Doesn't exist? */
+                                return log_debug_errno(errno, "Failed to open '%s': %m", lc->path);
                 }
 
                 if (sub_fd < 0)