Fix state is not reset when feature flag API request fails

[nwmac]

Summary

Fixes #11167

Occurred changes and/or fixed issues

This PR fixes the issue above and adds an e2e test to cover the changes.

We now revert the change to the feature flag model if the API request to update it fails, ensuring it does not appear changed in the UI.

We also improve the error message, taking the message from the error response if available.

Finally, this PR fixes a small bug, where if an error occurred, closing the dialog and clicking enable/disable again on a flag would show the previous error.

Note: We can't wire this into the flags that require restart, as the API does not always return before it restarts, so the API call can error, but that can indicate that a restart has started.

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