Fix _fix_permissions error handling and list_node_files PermissionError

- _fix_permissions: capture stderr, check returncode, only set
  _permissions_fixed on success instead of silently marking as fixed
- list_node_files: wrap os.scandir in try-except to handle
  PermissionError gracefully

gns3server/compute/docker/docker_vm.py

@@ -772,11 +772,19 @@ class DockerVM(BaseNode):
                     ' && /gns3/bin/busybox chown {uid}:{gid} -R "{path}"'.format(
                         uid=os.getuid(), gid=os.getgid(), path=volume
                     ),
+                    stderr=asyncio.subprocess.PIPE,
                 )
             except OSError as e:
                 raise DockerError(f"Could not fix permissions for {volume}: {e}")
             await process.wait()
-            self._permissions_fixed = True
+            if process.returncode != 0:
+                stderr = (await process.stderr.read()).decode(errors="replace").strip()
+                log.error(
+                    "Failed to fix permissions on '%s' for container '%s': %s",
+                    volume, self._name, stderr or f"exit code {process.returncode}"
+                )
+            else:
+                self._permissions_fixed = True
 
     async def _start_vnc_process(self, restart=False):
         """