Idempotent-ise schema update script

If any ASes don't have an ID, the schema will fail, and then it will
error when trying to add the column again.

synapse/storage/schema/delta/30/as_users.py

@@ -20,7 +20,11 @@ logger = logging.getLogger(__name__)
 
 def run_upgrade(cur, database_engine, config, *args, **kwargs):
     # NULL indicates user was not registered by an appservice.
+    try:
         cur.execute("ALTER TABLE users ADD COLUMN appservice_id TEXT")
+    except:
+        # Maybe we already added the column? Hope so...
+        pass
 
     cur.execute("SELECT name FROM users")
     rows = cur.fetchall()