Fix update not working offline

[mdawsonuk]

Description

Fixes #6 by adding a --offline command line switch and improved error handling

Pull Request Checklist:

• [x] I have written tests which cover at least 80% of the code I have written
• [x] I have documented the code I have added where necessary
• [x] I have updated the command line arguments where necessary
• [x] I have tested that the code against the list of supported applications
• [x] I have met all of the guidelines set by this project's contribution rules. These can be found linked in the READ ME