Avoid crash during initial migration

As suggested by @elelay on
gpodder#32 (comment)

mygpo/users/checks.py

@@ -1,6 +1,6 @@
 from django.core.checks import register, Warning
 from django.db import connection
-from django.db.utils import OperationalError
+from django.db.utils import OperationalError, ProgrammingError
 
 
 SQL = """
@@ -29,8 +29,8 @@ def check_case_insensitive_users(app_configs=None, **kwargs):
             wid = 'users.W001'
             errors.append(Warning(txt, id=wid))
 
-    except OperationalError as oe:
-        if 'no such table: auth_user' in str(oe):
+    except (OperationalError, ProgrammingError) as oe:
+        if 'no such table: auth_user' in str(oe) or 'relation "auth_user" does not exist' in str(oe):
             # Ignore if the table does not yet exist, eg when initally
             # running ``manage.py migrate``
             pass