[FEATURE] Custom error message snack bar style

[benedict1986]

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The style of the snackbar in flutter_login is slightly different from the snackbar I am using in other places. For example, the background color of the snackbar in flutter_login is red, while in other places, only the boarder color is red and the background color is white. I would like to make the error message style to be consistent. It would be nice that flutter_login can custom the snackbar style.

Thank you

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