Adds code to make the error returned when ISO validation fails more readable
A few small code changes to make the code more readable
Fixes references to "verify" methods from elsewhere in the codebase (code that prints metadata associated with an image to the command line and code that returns the default MK image reference)
Merge pull request #88 from tjmcs/tb/fix-iso-validation-error

Thanks, Tom.

Fixes #87 ('Missing metadata' thrown during ISO validation)