Check if model file is valid prior to starting workflow

[adelavega]

To avoid confusion like this: https://neurostars.org/t/fitlins-cant-use-the-model-fitlins-example-rhyme-judgment/20030

I think it would be better if we explicitly check for the file's existence and spit out a nicer error.

[shashankbansal6]

++1 on a nicer error for sure. I think I was seeing this exact error in one of the fitlins tests and the issue took me a while to understand. I did add a slightly better error message in the refactor:

https://github.com/poldracklab/fitlins/blob/747c91441f4e34eed31be07683e02af0e2700bfb/fitlins/cli/run.py#L270-L271