Add plug to generate Matrix error

lib/matrix_server_web/plug/error.ex

@@ -0,0 +1,23 @@
+defmodule MatrixServerWeb.Plug.Error do
+  import Plug.Conn
+  import Phoenix.Controller, only: [json: 2]
+
+  @error_code_and_message %{
+    bad_json: {400, "M_BAD_JSON", "Bad request."},
+    user_in_use: {400, "M_USE_IN_USE", "Username is already taken."},
+    invalid_username: {400, "M_INVALID_USERNAME", "Invalid username."},
+    forbidden: {400, "M_FORBIDDEN", "The requested action is forbidden."},
+    unknown_token: {401, "M_UNKNOWN_TOKEN", "Invalid access token."},
+    missing_token: {401, "M_MISSING_TOKEN", "Access token required."}
+  }
+
+  def put_error(conn, error) do
+    {status, errcode, errmsg} = @error_code_and_message[error]
+    data = %{errcode: errcode, error: errmsg}
+
+    conn
+    |> put_status(status)
+    |> json(data)
+    |> halt()
+  end
+end