Fail the build if the models are missing

[kamilzyla]

Problem

To build Mbaza, one needs to download the ONNX models and place them in the assets/models directory. However, there is no warning/failure during build if the files are missing or misnamed (like a (1) suffix added by web browser when downloading the model).

Goal

Check during the build process whether all the necessary models are present in the assets/models directory and fail the build otherwise.