[Gazebo] Rework nav bar error state

[spalmurray-codecov]

Currently, if something in the nav bar errors out, the error state looks like this:

This is probably not ideal. We should rethink the error state. Perhaps just a smaller message? Maybe a toast?

[suejung-sentry]

I hid the top Header for now in the case of error instead of the jarring upside down umbrella in here - codecov/gazebo#3590.
But it would still be nice to get some design input if a Toast or other message is better to display when specific parts of the header have network call errors.