occured -> occurred (#660)

* Fix duplicate source string

* occured -> occurred

All resources I could find listed this as misspelled, but I wasn't too
sure…
Also, it should be checked if all changes to the .po-files are respected
in the corresponding src-files.

src/pretix/base/services/orders.py

@@ -53,7 +53,7 @@ error_messages = {
                  'the quantity you selected. Please see below for details.'),
     'price_changed': _('The price of some of the items in your cart has changed in the '
                        'meantime. Please see below for details.'),
-    'internal': _("An internal error occured, please try again."),
+    'internal': _("An internal error occurred, please try again."),
     'empty': _("Your cart is empty."),
     'max_items_per_product': _("You cannot select more than %(max)s items of the product %(product)s. We removed the "
                                "surplus items from your cart."),

src/pretix/base/views/async.py

@@ -133,7 +133,7 @@ class AsyncAction:
             return str(exception)
         else:
             logger.error('Unexpected exception: %r' % exception)
-            return _('An unexpected error has occured.')
+            return _('An unexpected error has occurred.')
 
     def get_success_message(self, value):
         return _('The task has been completed.')