Fix crash on invalid rails6-format cookie

[guisehn]

When providing an invalid cookie in Rails 6 format (with 3 parts), MessageEncryptor.authenticate_and_decrypt/2 would return {:ok, :error} instead of {:error}, which would crash on:

Request: GET /
** (exit) an exception was raised:
    ** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not an iodata term

        :erlang.iolist_to_binary(:error)
        (jason 1.3.0) lib/jason.ex:69: Jason.decode/2
        (plug_rails_cookie_session_store 2.0.0) lib/plug_rails_cookie_session_store.ex:171: PlugRailsCookieSessionStore.decode/2
        (plug 1.13.6) lib/plug/session.ex:78: anonymous fn/5 in Plug.Session.fetch_session/1
        (plug 1.13.6) lib/plug/debugger.ex:287: Plug.Debugger.maybe_fetch_session/1
        (plug 1.13.6) lib/plug/debugger.ex:187: Plug.Debugger.render/6
        (plug 1.13.6) lib/plug/debugger.ex:162: Plug.Debugger.__catch__/5

We fixed to return just :error in that case, which gracefully falls back to an empty session.