Improve the Unsupported Block Editor offline experience

[guarani]

Related: #2358

Describe the bug
The Unsupported Block Editor requires an internet connection to function, but does not provide any indication to the user if their internet connection is offline.

To Reproduce
Steps to reproduce the behavior:

1. On the WordPress for iOS or Android apps, edit a post containing a block not currently supported by the mobile Block editor.
2. Click the (?) button on the unsupported block placeholder
3. Turn off the device's internet connection
4. Tap the "Edit block in web browser" option in the bottom sheet that appeared in Step 2
5. Notice the screen loads for a short time before staying blank, providing no indication that there is no internet connection

Expected behavior
Upon opening the Unsupported Block Editor, if there is no internet connection, the UI should communicate this is some way.

Screenshots

Smartphone (please complete the following information):

• Device: iPhone 7 or any other supported device
• OS: iOS 14 or any other OS
• Version: This feature is soon-to-be-released in WPiOS and WPAndroid version 15.3.

[hypest]

Now that we have the intermediate loading screen in place, I wonder if we could use it for notifying of no-network too. WDYT @guarani?

[guarani]

👋 @hypest, I agree this makes sense now that we have the loading screen in place.

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@SiobhyB regarding the implementation, on iOS I'd go with using WPiOS' existing offline screen. Given our recent discussion about the differences between implementations on iOS and Android, I wanted to check that using this screen from the main apps is a good approach overall.
(I'm not suggesting we're necessarily going to work on this, just was curious about your opinion here given your recent work in this part of the app.)

[guarani]

iOS	Android

These are the two offline screens I saw in the main app (they're not used everywhere though).