- [x] Catch SocketErrors and handle them gracefully.
- [x] Print 'Lost connection to device' when the service protocol connection is severed unexpectedly.
- [x] Print 'Application finished' when the application exits otherwise.

After this PR:

```
Launching lib/main.dart on Nexus 7 in debug mode...
Running 'gradle assembleDebug'...                     1.2s
Built build/app/outputs/apk/app-debug.apk (21.7MB).
Syncing files to device...
Application finished.
DevFS sync failed. Lost connection to device: SocketException: OS Error: Connection refused, errno = 111, address = 127.0.0.1, port = 53062
Could not perform initial file synchronization.
```

Fixes #6705