sys.exit rather than throwing an assertion error

Assertions should only be used in tests - they can be disabled at
runtime by setting the python flag -O (though I don't believe we use
that flag under normal circumstances).

also clean up test asserts - mock_redis is the redis object, so its
`called` property will always be false, because we never say
`redis_store()`. Rather, we should use the `mock_calls` property to
see all calls to all of its children

app/commands.py

@@ -1,3 +1,4 @@
+import sys
 import functools
 import uuid
 from datetime import datetime, timedelta
@@ -504,9 +505,11 @@ def populate_redis_template_usage(service_id, day):
     Recalculate and replace the stats in redis for a day.
     To be used if redis data is lost for some reason.
     """
-    # the day variable is set to midnight of that day
-    assert current_app.config['REDIS_ENABLED']
+    if not current_app.config['REDIS_ENABLED']:
+        current_app.logger.error('Cannot populate redis template usage - redis not enabled')
+        sys.exit(1)
 
+    # the day variable is set by click to be midnight of that day
     start_time = get_london_midnight_in_utc(day)
     end_time = get_london_midnight_in_utc(day + timedelta(days=1))
 
@@ -533,5 +536,6 @@ def populate_redis_template_usage(service_id, day):
         redis_store.set_hash_and_expire(
             key,
             usage,
-            current_app.config['EXPIRE_CACHE_EIGHT_DAYS']
+            current_app.config['EXPIRE_CACHE_EIGHT_DAYS'],
+            raise_exception=True
         )