abstract error/checking parts of greta

[njtierney]

For example, I'm reasonably sure these lines

https://github.com/greta-dev/greta/blob/master/R/greta_model_class.R#L87-L172

Could be abstracted into some general checking function that checks whether nodes are not connected into each other. This would help make some of the code a bit tighter, but is a low priority.

[njtierney]

I'm currently writing a lot of checking functions into descriptive checker functions that are in the checkers.R file. My process is that these can be refactored as time goes on, but the important thing is to make the function body of things in greta read more clearly. Sometimes the checking code can be complex and it interrupts understanding the intent of the function.

[njtierney]

A few more points about error checking code in greta

1. They should use cli::cli_abort() instead of the current pattern of constructing a message then passing it to stop
2. checking functions should have arg and call arguments, as discussed in the rlang docs
3. "explainable variables" or short functions should be used to help simplify the if statements in error checking

I came across these as I was writing about the error checking code smell in a blog post, Code Smell: Error Handling Eclipse