Added a Custom Exception and supporting tests if a model file is not found.

[exula]

Hello there, Here is a pull request that adds a custom exception if the Model Data file is not found, and it uses the Flare Custom Solutions API to suggest the command to run to fix the problem.

I've included a test of an Untrained model also, just as a quick way to get the exception to throw. https://flareapp.io/docs/solutions/adding-custom-solutions

Hope this helps!