Make move_account endpoint process non-existent users properly

Ref: emit-move

test/pleroma/web/twitter_api/util_controller_test.exs

@@ -573,6 +573,25 @@ defmodule Pleroma.Web.TwitterAPI.UtilControllerTest do
              }
     end
 
+    test "with proper permissions, valid password and target account does not exist",
+         %{
+           conn: conn
+         } do
+      target_nick = "not_found@mastodon.social"
+
+      conn =
+        conn
+        |> put_req_header("content-type", "multipart/form-data")
+        |> post("/api/pleroma/move_account", %{
+          "password" => "test",
+          "target_account" => target_nick
+        })
+
+      assert json_response_and_validate_schema(conn, 404) == %{
+               "error" => "Target account not found."
+             }
+    end
+
     test "with proper permissions, valid password and target account aliases this", %{
       conn: conn,
       user: user

test/support/http_request_mock.ex

@@ -725,6 +725,15 @@ defmodule HttpRequestMock do
      }}
   end
 
+  def get(
+        "https://mastodon.social/.well-known/webfinger?resource=acct:not_found@mastodon.social",
+        _,
+        _,
+        [{"accept", "application/xrd+xml,application/jrd+json"}]
+      ) do
+    {:ok, %Tesla.Env{status: 404}}
+  end
+
   def get("http://gs.example.org/.well-known/host-meta", _, _, _) do
     {:ok,
      %Tesla.Env{