Add exception and comments to explain the issue

lib/better_errors/error_page_style.rb

@@ -2,7 +2,20 @@ module BetterErrors
   # @private
   module ErrorPageStyle
     def self.compiled_css(for_deployment = false)
-      require "sassc"
+      begin
+        require "sassc"
+      rescue LoadError
+        raise LoadError, "The `sassc` gem is required when developing the `better_errors` gem. "\
+          "If you're using a release of `better_errors`, the compiled CSS is missing from the released gem"
+        # If you arrived here because sassc is not in your project's Gemfile,
+        # the issue here is that the release of the better_errors gem
+        # is supposed to contain the compiled CSS, but that file is missing from the release.
+        # So better_errors is trying to build the CSS on the fly, which requires the sassc gem.
+        #
+        # If you're developing the better_errors gem locally, and you're running a project
+        # that does not have sassc in its bundle, run `rake style:build` in the better_errors
+        # project to compile the CSS file.
+      end
 
       style_dir = File.expand_path("style", File.dirname(__FILE__))
       style_file = "#{style_dir}/main.scss"