Please allow me to override the gutter error popup decoration

[sma]

Please allow me to override the gutter error popup decoration. Currently, it's hardcoded in lib/src/gutter/error.dart and cannot be changed because I cannot provide my own GutterErrorWidget, or even better, provide a shape and/or borderSide using the GutterStyle. Providing a popupBuilder would be an alternative.

I'm using version 0.2.15.