Add hashbang, script description, and useful error message to convert_db script

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

convert_db.py

@@ -1,3 +1,7 @@
+#!/usr/bin/env python3
+# Convert between the old database format and the new one.
+# You probably won't need to run this script.
+
 import json
 import sqlite3
 
@@ -52,6 +56,10 @@ bans = old_db.execute("SELECT * FROM BANS").fetchall()
 print("Converting", len(bans), "rows")
 
 for ban in bans:
+    if 'post' not in ban:
+        print("ERROR: The database either does not exist, or already exists in the new format. Exiting")
+        raise SystemExit(1)
+
     post = json.loads(ban["post"])
     post["action"] = ban["action"]
     # post['board'] = ban['board']