Extreme scan tests

Main changes:

• ErrorHandler class in extreme.py
• Tests for extreme.py

I added new class ErrorHandler to extreme.py. This class is responsible for checking the config files and raising MessageBox with description of the error. User can choose to Retry to load a different config file or just cancel the program.

The tests are designed to run through different configs to check the edge cases:

• config_device_mode_all_filled → ideally filled config
• config_device_mode_no_entry → missing entries → use of hints
• config_scan_mode → different config for different incoming scans
• config_all_wrong → missing critical fields

The main app and error handler is tested separately. One reason is that at this point I do not know how to mock the messageBox properly, the second reason is that at some point I want to generalise ErrorHandler to show any errors from the future apps.

This merge request consider issues #8 (closed) and #4 (closed) .