Detect offline hub children and fail fast on writes to them (#1830)

Hubs keep reporting for children they have lost contact with, so callers
see stale values as if they were live. Trying to write to such devices
fail after timeouts with a generic error.
This PR exposes the offline/online information as reported by the hub,
and refuses writes to unreachable child devices without waiting for
device errors.

In details:
- Add new API: `Device.is_available`; hub children report unavailable
when the hub marks them offline or no longer lists them.
- New feature `available` for child devices to make it visible to the
CLI.
- Refuse writes to unavailable children with the new
`DeviceUnavailableError` instead of waiting for the hub to time out.

kasa/exceptions.py

@@ -55,6 +55,10 @@ class AuthenticationError(DeviceError):
     """Base exception for device authentication errors."""
 
 
+class DeviceUnavailableError(DeviceError):
+    """Exception for commands to an unreachable device."""
+
+
 class _RetryableError(DeviceError):
     """Retryable exception for device errors."""