MultiApiController: fix on_validation_error() to set code

Some errors don't have a code associated with them so we need to
supply a default value to abort_with_error. This fixes the
USER_DOESNT_EXIST errors that are causing exceptions.

r2/r2/controllers/multi.py

@@ -88,7 +88,8 @@
 
 
 class MultiApiController(RedditController):
-    on_validation_error = staticmethod(abort_with_error)
+    def on_validation_error(self, error):
+        abort_with_error(error, error.code or 400)
 
     def pre(self):
         set_extension(request.environ, "json")