Parent #13326

This PR displays full screen loading and errors on fetch scan and start scan requests.

Full screen loading/ errors

Loading (0)	No Network (1)	Generic Request Failed (3)	Start Scan Request Failed (4)

Snack bars

No Network (5)	Generic Request Failed (6)

To test

Loading (0), No Network (1)

1. Fresh install the app and logon to wp.com account with site having scan capability
2. Switch to site with scan capability
3. Enable airplane mode
4. Click Scan menu
5. Notice that fullscreen "Loading scan" is shown, followed by full screen "No network available" message
6. Disable airplane mode
7. Click retry
8. Notice that scan content is shown

No network snackbar (5)

1. Continuing from above, while being on the scan screen, enable airplane mode
2. Click scan button
3. Notice that a snackbar is shown with no network available message

Generic Request Failed (3), Start Scan Request Failed (4), Generic Request Failed snackbar (6)

These states can be tested by using
ScanViewModel tests:

Generic Request Failed (3)
given fetch scan fails, when scan state fetched over empty scan state, then request failed ui state shown
given request failed error ui state, when contact support is clicked, then contact support screen is shown

Start Scan Request Failed (4)
given scan start request fails, when scan button is clicked, then request failed state is shown
given start scan request failed error state, when contact support is clicked, then contact support shown

Generic Request Failed snackbar (6)
given fetch scan state fails, when scan state fetched over last scan state, then request failed msg shown

Notes:

1. Start scan request fail displays a full screen error with contact support button, message string is copied from the iOS app.
2. I plan to implement pull-to-refresh which will enable users to retry fetch scan state on full screen errors.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.