DEV: Improve postcss error handling (#31420)

Followup to 087e8e4bdb53f71930ec5c930c463c37dd2bd58d

- Fixes the variable-prefixer so it doesn't explode when the input is
unparseable
- Add URL polyfills so that postcss can print its errors properly
- Catch postcss errors in the same way as sass errors

lib/stylesheet/manager/builder.rb

@@ -49,7 +49,7 @@ class Stylesheet::Manager::Builder
           load_paths: load_paths,
           dark: @dark,
         )
-      rescue SassC::SyntaxError, SassC::NotRenderedError => e
+      rescue SassC::SyntaxError, SassC::NotRenderedError, DiscourseJsProcessor::TranspileError => e
         if Stylesheet::Importer::THEME_TARGETS.include?(@target.to_s)
           # no special errors for theme, handled in theme editor
           ["", nil]