Redstone exception mapper plugin

Usage

• Add redstone_exception_mapper to your pubspec.yaml.
• Import the plugin import 'package:redstone_exception_mapper/redstone_exception_mapper.dart' as plugin;
• Add this line before the setup() call of Redstone : app.addPlugin(plugin.ExceptionMapperPlugin);
• (Optional) You may turn off Redstone's error pages : app.showErrorPage = false;
• Map your exceptions to a function returning a new shelf Response.

  plugin.mappings = {
    NotFoundException: (NotFoundException e) => new Response.notFound(e.message)
  };