PIA-1920: Introduce DIP signup error handling

[kp-juan-docal]

Summary

As per title. It introduces the error handling for the majority of cases. Those being:

• The Plans failed to be fetched for the first time.
• The list of supported countries failed to be fetched for the first time.
• The Billing library purchase failed.
• The confirmation of the purchase with our backend failed.

Sanity Tests

Done by mocking responses

• [x] Open the app. Mock-fail the request to get the list of supported countries. Confirm we show an error and abort the purchase.
• [x] Open the app. Mock-fail the request to get the subscriptions. Confirm we show an error and abort the purchase.
• [x] Open the app. Mock-fail the billing library payment. Confirm we show an error and abort the purchase.
• [x] Open the app. Mock-fail the confirmation of purchase with our backend. Confirm we show an error and abort the purchase.