Raise exception if data is not available when update is called

This happens if the first auction is unsuccessful.

Implemented by verifying that the data is valid (in
join_result_for_correct_time()) and raising an exception if it isn't.

This partially fixes issue custom-components#326.

custom_components/nordpool/aio_price.py

@@ -90,6 +90,12 @@
     "PL ": "PL",
 }
 
+INVALID_VALUES = frozenset((None, float("inf")))
+
+
+class InvalidValueException(ValueError):
+    pass
+
 
 def join_result_for_correct_time(results, dt):
     """Parse a list of responses from the api
@@ -135,6 +141,8 @@ def join_result_for_correct_time(results, dt):
                 local = val["start"].astimezone(zone)
                 local_end = val["end"].astimezone(zone)
                 if start_of_day <= local and local <= end_of_day:
+                    if val['value'] in INVALID_VALUES:
+                        raise InvalidValueException()
                     if local == local_end:
                         _LOGGER.info(
                             "Hour has the same start and end, most likly due to dst change %s exluded this hour",