Refactor CLI usage into CLI.run!:

* Add Producer::Core::CLI::ArgumentError class;
* Raise ArgumentError in CLI.new when recipe is missing;
* Remove #check_arguments! and #print_usage_and_exit, move code into
  .run! method.

lib/producer/core/cli.rb

@@ -1,24 +1,34 @@
 module Producer
   module Core
     class CLI
+      ArgumentError = Class.new(::ArgumentError)
+
       USAGE = "Usage: #{File.basename $0} recipe_file"
 
+      class << self
+        def run!(arguments, output: $stderr)
+          begin
+            cli = new(arguments)
+          rescue ArgumentError
+            output.puts USAGE
+            exit 64
+          end
+          cli.run!
+        end
+      end
+
       attr_reader :arguments, :stdout
 
       def initialize(arguments, stdout: $stdout)
+        raise ArgumentError unless arguments.any?
         @arguments  = arguments
         @stdout     = stdout
       end
 
       def run!
-        check_arguments!
         interpreter.process recipe.tasks
       end
 
-      def check_arguments!
-        print_usage_and_exit(64) unless @arguments.length == 1
-      end
-
       def env
         @env ||= Env.new
       end
@@ -30,14 +40,6 @@ module Producer
       def interpreter
         @interpreter ||= Interpreter.new
       end
-
-      private
-
-      def print_usage_and_exit(status)
-        @stdout.puts USAGE
-
-        exit status
-      end
     end
   end
 end