management: Don't use sys.exit(1).

Using sys.exit in a management command makes it impossible
to unit test the code in question.  The correct approach to do the same
thing in Django management commands is to raise CommandError.

Followup of b570c0dafa

analytics/management/commands/clear_single_stat.py

@@ -1,8 +1,7 @@
-import sys
 from argparse import ArgumentParser
 from typing import Any
 
-from django.core.management.base import BaseCommand
+from django.core.management.base import BaseCommand, CommandError
 
 from analytics.lib.counts import COUNT_STATS, do_drop_single_stat
 
@@ -20,10 +19,8 @@ class Command(BaseCommand):
     def handle(self, *args: Any, **options: Any) -> None:
         property = options['property']
         if property not in COUNT_STATS:
-            print("Invalid property: %s" % (property,))
-            sys.exit(1)
+            raise CommandError("Invalid property: %s" % (property,))
         if not options['force']:
-            print("No action taken. Use --force.")
-            sys.exit(1)
+            raise CommandError("No action taken. Use --force.")
 
         do_drop_single_stat(property)