Fallback server errors rather than starting

[julianstirling]

Describe the bug
The fallback server shows an internal server error if you try to see what the error is. This is because the config is a ThingServerConfig but we try to dump it with json.dumps. The tests pass as the fallback tests don't pass in any config.

To Reproduce

Create conf.json with:

{
    "things": {
        "example": "labthings_fastapi.example_things:ThingThatCantStart"
    }
}

Run:
labthings-server --fallback -c conf.json

Looks like the fallback server is fine until you go to 127.0.0.1:5000, where you get an internal server error

Expected behaviour
No error

System:

• OS: Kubunto 24.04
• Python version: 3.11
• Version: 0.0.13