Show a relative path when describing configuration missing

For a mounted Rails engine we need the config to be in the application
mount directory e.g. test/dummy/config/webpacker.yml

By running this based on pwd we get this in a Rails engine app:

```
➜  rails-plugin rake app:webpacker:verify_install
RAILS_ENV=development environment is not defined in config/webpacker.yml, falling back to production environment
Configuration test/dummy/config/webpacker.yml file not found.
Make sure webpacker:install is run successfully before running dependent tasks
```

Whereas in a normal Rails installation it outputs unchanged:

```
➜  full-app git:(master) ✗ rake webpacker:verify_install
RAILS_ENV=development environment is not defined in config/webpacker.yml, falling back to production environment
Configuration config/webpacker.yml file not found.
Make sure webpacker:install is run successfully before running dependent tasks
```

lib/tasks/webpacker/verify_install.rake

@@ -4,7 +4,8 @@ namespace :webpacker do
   desc "Verifies if Webpacker is installed"
   task verify_install: [:check_node, :check_yarn, :check_binstubs] do
     unless Webpacker.config.config_path.exist?
-      $stderr.puts "Configuration config/webpacker.yml file not found. \n"\
+      path = Webpacker.config.config_path.relative_path_from(Pathname.new(pwd)).to_s
+      $stderr.puts "Configuration #{path} file not found. \n"\
            "Make sure webpacker:install is run successfully before " \
            "running dependent tasks"
       exit!