```ruby
require 'middleman-core/cli'
require 'middleman-s3_sync/extension'

module Middleman
  module Cli
    class S3Sync < Thor
      include Thor::Actions
      namespace :s3_sync

      def self.exit_on_failure?
        true
      end

      desc "s3_sync", "Pushes the minimum set of files needed to S3"
      option :force, type: :boolean,
                     desc: "Push all local files to the server",
                     aliases: :f
      option :bucket, type: :string,
                      desc: "Specify which bucket to use, overrides the configured bucket.",
                      aliases: :b
      option :verbose, type: :boolean,
                       desc: "Adds more verbosity...",
                       aliases: :v

      def s3_sync
        shared_inst = ::Middleman::Application.server.inst
        bucket = shared_inst.s3_sync_options.bucket
        if (!bucket)
          raise Thor::Error.new "You need to activate this extension."
        end

        # Override options based on what was passed on the command line...
        shared_inst.s3_sync_options.force = options[:force] if options[:force]
        shared_inst.s3_sync_options.bucket = options[:bucket] if options[:bucket]
        shared_inst.s3_sync_options.verbose = options[:verbose] if options[:verbose]

        ::Middleman::S3Sync.sync
      end
    end
  end
end
```

Return nil if the extension isn't loaded, update the error message.
user
```ruby
require 'middleman-core/cli'
require 'middleman-s3_sync/extension'

module Middleman
  module Cli
    class S3Sync < Thor
      include Thor::Actions
      namespace :s3_sync

      def self.exit_on_failure?
        true
      end

      desc "s3_sync", "Pushes the minimum set of files needed to S3"
      option :force, type: :boolean,
                     desc: "Push all local files to the server",
                     aliases: :f
      option :bucket, type: :string,
                      desc: "Specify which bucket to use, overrides the configured bucket.",
                      aliases: :b
      option :verbose, type: :boolean,
                       desc: "Adds more verbosity...",
                       aliases: :v

      def s3_sync
        shared_inst = ::Middleman::Application.server.inst
        bucket = shared_inst.s3_sync_options.bucket rescue nil
        unless bucket
          raise Thor::Error.new "You need to activate the s3_sync extension."
        end

        # Override options based on what was passed on the command line...
        shared_inst.s3_sync_options.force = options[:force] if options[:force]
        shared_inst.s3_sync_options.bucket = options[:bucket] if options[:bucket]
        shared_inst.s3_sync_options.verbose = options[:verbose] if options[:verbose]

        ::Middleman::S3Sync.sync
      end
    end
  end
end
```
assistant