Ensure Scratch Link connection recovers from flaky network

[cwillisf]

Expected Behavior

• If the extension system tries to connect to Scratch Link and the DNS lookup fails, the Scratch Link connection as a whole will fail.
• If the network connects after this initial failure, possibly because flaky WiFi reconnected or the user connected to a new network, the connection to Scratch Link should start working. Ideally this should work without reloading the editor page.
• Error status information should also be displayed to the user as appropriate.

Actual Behavior

• If the extension system tries to connect to Scratch Link and the DNS lookup fails, the Scratch Link connection as a whole will fail.
• If the network connects after this initial failure, possibly because flaky WiFi reconnected or the user connected to a new network, the connection to Scratch Link should start working. Ideally this should work without reloading the editor page.
  • Current reports suggest that this sometimes work and sometimes doesn't.
• Error status information should also be displayed to the user as appropriate.
  • There may be cases where we are missing feedback.
  • It would be nice to find a way to distinguish between "Scratch Link is not running" and "Cannot resolve device-manager.scratch.mit.edu

Steps to Reproduce

1. Start Scratch Link
2. Start Scratch 3.0 in any form
3. Disconnect Internet access
4. Load a hardware extension
5. Attempt to connect the extension to a peripheral - this should fail
6. Reconnect Internet access (do not restart or reload Scratch)
7. Attempt to connect the extension to a peripheral - this should succeed