validate PseudoOps files when provided by the user

[MoralCode]

this is needed to prevent the app from being unable to start up if the user selects a bad file (like a PDF) that's clearly in the wrong format

maybe check for text file types? How much is too much or too little validation?
would "is a text file" and "correctly loads into InstructionSet" enough? or should there be a warning if any single line within the file is not a valid pseudo op?