Filepath validation for artifact_filepath

[Samkit-shah]

https://github.com/JacksonQu/Software-Supply-Chain-Security-Assignment1/blob/d446488fdbda84f8264478efa141aa9ba9dba792/main.py#L50

There is currently no check to verify whether the artifact_filepath actually points to a valid file. It's important to add this validation because, in theverify_artifact_signature function from the util file, the file is directly opened without checking if it exists or is accessible. This could result in the function breaking if the file is invalid or missing.

[JacksonQu]

Fixed in branch origin/fix-issue-5