improve error message if no apps found in registry (#2585)

tests/test_app.py

@@ -347,7 +347,13 @@ def test_app_registry_retrieval_from_multiple():
 
 def test_get_app_does_not_exist():
     with pytest.raises(
-        SanicException, match='Sanic app name "does-not-exist" not found.'
+        SanicException,
+        match="Sanic app name 'does-not-exist' not found.\n"
+            "App instantiation must occur outside "
+            "if __name__ == '__main__' "
+            "block or by using an AppLoader.\nSee "
+            "https://sanic.dev/en/guide/deployment/app-loader.html"
+            " for more details."
     ):
         Sanic.get_app("does-not-exist")