EntityComponent to retry platforms that are not ready yet (#8209)

* Add PlatformNotReady Exception

* lint

* Remove cap, adjust algorithm

homeassistant/exceptions.py

@@ -26,3 +26,9 @@ class TemplateError(HomeAssistantError):
         """Init the error."""
         super().__init__('{}: {}'.format(exception.__class__.__name__,
                                          exception))
+
+
+class PlatformNotReady(HomeAssistantError):
+    """Error to indicate that platform is not ready."""
+
+    pass