manager: emit a message when we fail to create manager because /run is not set up

$ SYSTEMD_LOG_LEVEL=debug build/systemd --test --user
...
Failed to lookup RuntimeDirectory path: No such device or address   <---- this line is new
Failed to allocate manager object: No such device or address

We would fail and only say "Failed to allocate manager object: ENODEV" which is
not entirely self-explanatory. Let's add a better log message.

src/core/manager.c

@@ -688,7 +688,8 @@ static int manager_setup_prefix(Manager *m) {
         for (ExecDirectoryType i = 0; i < _EXEC_DIRECTORY_TYPE_MAX; i++) {
                 r = sd_path_lookup(p[i].type, p[i].suffix, &m->prefix[i]);
                 if (r < 0)
-                        return r;
+                        return log_warning_errno(r, "Failed to lookup %s path: %m",
+                                                 exec_directory_type_to_string(i));
         }
 
         return 0;