bestellanforderung: Improve error message for unknown accounts

bestellanforderung.py

@@ -191,7 +191,8 @@ def scrape_teilbescheinigung(filename, narration, date, accounts):
 
     account = accounts.get(d.get('psp') or d.get('cost-center') or d.get('funds-center'))
     if not account:
-        raise ValueError(f'unknown account in {filename}')
+        keys = ', '.join(d.get(x) for x in ('psp', 'cost-center', 'funds-center'))
+        raise ValueError(f'unknown account in {filename}: {keys}')
 
     # The form does not contain amount information.
     amount = data.Amount.from_string('0.00 EUR')