write warnings for invalid models

[davidtedfordholt]

I'm not entirely sure that we will be able to warn for all of them, but if we find model specifications that don't make sense or don't run we need to document them and create warnings for them. It would be nice if those warnings just returned an empty mbl object instead of erroring the process out.

[mitchelloharawild]

I'm not sure what you're looking for with an empty mbl object, something like the fabletools::null_model()?

Preventing one modelling error from stopping the process is handled in fabletools::model() via the .safely argument (default TRUE). If a model errors during training, a null_model() will be returned and a the error will be converted to a warning.

[davidtedfordholt]

I’m always happy to know that a local problem already has a general solution. Thank you! I’ll focus solely on problematically misspecified models.