Raise a `ValidationError` for positional-only plans

[callumforrester]

So we only raise a TypeError when we try and run it? We don't raise a ValidationError while making the model? Can we make a ticket and change this behaviour

Originally posted by @DiamondJoseph in #1184 (comment)

We have documented the behaviour when trying to use positional-only arguments in plans #780. However the error is only thrown when trying to run a plan. It should be thrown when loading the plan on startup.