Create action_fallback for username/password incorrect input

lib/pleroma/web/oauth/fallback_controller.ex

@@ -0,0 +1,19 @@
+defmodule Pleroma.Web.OAuth.FallbackController do
+    use Pleroma.Web, :controller
+    alias Pleroma.Web.OAuth.OAuthController
+
+    # No user
+    def call(conn, nil) do
+        conn
+        |> put_flash(:error, "Invalid Username/Password")
+        |> OAuthController.authorize(conn.params)
+    end
+
+    # No password
+    def call(conn, false) do
+        conn
+        |> put_flash(:error, "Invalid Username/Password")
+        |> OAuthController.authorize(conn.params)
+    end
+
+end