Add more descriptive error to Observer

[Raild3x]

Currently if you give a scope, but not a state to watch, the Observer errors out telling you that your scope is missing. This led me on a goose chase trying to figure out why it wasn't able to find my scope. Turned out it wasn't the scope, but the state that was missing. This change aims to just adjust the error message displayed in this edge case. I wasn't sure how best to log the error, feel free to adjust.