RunCellpose warning for custom model not found

[fefossa]

Is your feature request related to a problem? Please describe.

Running RunCellpose with a custom model, and changing the model from the previously defined folder, doesn't warn you that the file is not found anymore; the module just runs with a built-in model from cellpose, without raising any error.

Describe the solution you'd like

Raise ValidationError in the pipeline for File not found when running RunCellpose with custom models.