Poor messaging when new image types added

[Deijin27]

If someone has previously run "Populate Graphics Defaults" and then runs a later version of ranseilink which adds new image types, the error message that occurs is not clear to them.

Currently it only checks if the defaults folder exists for notifying the user that they need to run "Populate graphics defaults", so what users currently see is just the file not found error

Should add a more thorough check to make sure every file exists so we can give the more useful error message all of the time.