MastodonUI login: Show error message on error.

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -559,7 +559,7 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIController do
 
   def login(conn, _) do
     conn
-    |> render(MastodonView, "login.html")
+    |> render(MastodonView, "login.html", %{error: false})
   end
 
   defp get_or_make_app() do
@@ -581,6 +581,10 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIController do
       conn
       |> put_session(:oauth_token, token.token)
       |> redirect(to: "/web/getting-started")
+    else
+      _e ->
+        conn
+        |> render(MastodonView, "login.html", %{error: "Wrong username or password"})
     end
   end
 

lib/pleroma/web/templates/mastodon_api/mastodon/login.html.eex

@@ -1,4 +1,7 @@
 <h2>Login in to Mastodon Frontend</h2>
+<%= if @error do %>
+  <h2><%= @error %></h2>
+<% end %>
 <%= form_for @conn, mastodon_api_path(@conn, :login), [as: "authorization"], fn f -> %>
 <%= text_input f, :name, placeholder: "Username" %>
 <br>