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.

docs/source/reference.md

@@ -122,6 +122,12 @@
     :undoc-members:
 ```
 
+```{eval-rst}
+.. autoclass:: kasa.exceptions.DeviceUnavailableError
+    :members:
+    :undoc-members:
+```
+
 ```{eval-rst}
 .. autoclass:: kasa.exceptions.UnsupportedDeviceError
     :members:

docs/source/topics.md

@@ -137,6 +137,8 @@ The base exception for all library errors is {class}`KasaException <kasa.excepti
 - If the device returns an error the library raises a {class}`DeviceError <kasa.exceptions.DeviceError>` which will usually contain an ``error_code`` with the detail.
 - If the device fails to authenticate the library raises an {class}`AuthenticationError <kasa.exceptions.AuthenticationError>` which is derived
   from {class}`DeviceError <kasa.exceptions.DeviceError>` and could contain an ``error_code`` depending on the type of failure.
+- If a command is sent to a hub child that is not available (see {attr}`Device.is_available <kasa.Device.is_available>`) the library raises a {class}`DeviceUnavailableError <kasa.exceptions.DeviceUnavailableError>`,
+  derived from {class}`DeviceError <kasa.exceptions.DeviceError>`, without contacting the device.
 - If the library encounters and unsupported deviceit raises an {class}`UnsupportedDeviceError <kasa.exceptions.UnsupportedDeviceError>`.
 - If the device fails to respond within a timeout the library raises a {class}`TimeoutError <kasa.exceptions.TimeoutError>`.
 - All other failures will raise the base {class}`KasaException <kasa.exceptions.KasaException>` class.