Added reauth flow, much better error handling on setup

### Feature
* Now supports reauthentication if credentials have expired/no longer work
* Better error handling when configuring integration, will notify user of incorrect credentials

### Fix
* Removed Owlet specific constants, now using homeassistant generic constants
* On initialisation the integration would crash when trying to update the auth token, the integration would then have to be deleted and setup again

custom_components/__init__.py

@@ -0,0 +1 @@
+"""Custom Integrations for Home Assistant."""