Error message for MaterialDesignOutlinedTextBox should has same margin/padding as HelperText.

<TextBox Style="{StaticResource MaterialDesignOutlinedTextBox}" materialDesign:HintAssist.Hint="Category name*" materialDesign:HintAssist.HelperText="*Required"/> 

Side note: I think that the issue is connected with PR #2217 that fixed the padding specification on Filled or Outlined TextBox style. Unfortunately it doesn't cover validation template.