Reorganize LuauExecutionSessionTask into an enum to handle error states

[nightcycle]

In the Roblox documentation it specifies that the result can return an error field, or an output field. The struct is currently configured based on only the output field: https://github.com/Sleitnick/rbxcloud/blob/main/src/rbx/v2/luau_execution.rs

Currently when an error is returned there's a serde error: reqwest::Error { kind: Decode, source: Error("missing field output", line: 1, column: 368)

Using: v0.17.0