[firebase_ui_auth] allow custom handling of error text in SignInScreen

[CodePlayAdmin]

I hope that there are more ways to handle or show the error messages when authentication is failed in SignInScreen, such as a Snackbar or Dialog. Currently the error message is showed below the # / Register button as per the screenshot below:

This is sometime hardly to streamline the prompted error to a given UI design or behavior, and when the error message is long, it would become just like in this Stack Overflow post.