Catch KeyError on email_template default value

When installing the module on a database with existing procedures, trytond
tries to set the value for existing columns but it fails as the default
report template is not yet loaded on the database
issue6866
review39831002

account.py

@@ -42,7 +42,10 @@ class DunningLevel:
     def default_email_template(cls):
         pool = Pool()
         Data = pool.get('ir.model.data')
-        return Data.get_id('account_dunning_email', 'report_email')
+        try:
+            return Data.get_id('account_dunning_email', 'report_email')
+        except KeyError:
+            return
 
 
 class ProcessDunning: