lazy load app on reload only

CHANGES.rst

@@ -10,6 +10,9 @@ Unreleased
 -   Add :meth:`Config.from_file` to load config using arbitrary file
     loaders, such as ``toml.load`` or ``json.load``.
     :meth:`Config.from_json` is deprecated in favor of this. :pr:`3398`
+-   The ``flask run`` command will only defer errors on reload. Errors
+    present during the initial call will cause the server to exit with
+    the traceback immediately. :issue:`3431`
 
 
 Version 1.1.2