wip: error handling for network conditions

[rutajdash]

Tasks:

• Create a custom error screen for when the user is not connected to the internet.
• Add a check in app initialization and show error screen if required
• Watch for change in network status and show error screen when required

Additional Context

• https://pub.dev/packages/connectivity_plus
• https://stackoverflow.com/questions/49648022/check-whether-there-is-an-internet-connection-available-on-flutter-app