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.

spec/producer/core/cli_spec.rb

@@ -11,12 +11,50 @@ module Producer::Core
 
     subject(:cli)     { CLI.new(arguments, stdout: stdout) }
 
+    describe '.run!' do
+      let(:output)    { StringIO.new }
+      subject(:run)   { described_class.run! arguments, output: output }
+
+      it 'builds a new CLI with given arguments' do
+        expect(CLI).to receive(:new).with(arguments).and_call_original
+        run
+      end
+
+      it 'runs the CLI' do
+        cli = double 'cli'
+        allow(CLI).to receive(:new) { cli }
+        expect(cli).to receive :run!
+        run
+      end
+
+      context 'when recipe argument is missing' do
+        let(:arguments) { [] }
+
+        it 'exits with a return status of 64' do
+          expect { run }.to raise_error(SystemExit) { |e|
+            expect(e.status).to eq 64
+          }
+        end
+
+        it 'prints the usage' do
+          trap_exit { run }
+          expect(output.string).to match /\AUsage: .+/
+        end
+      end
+    end
+
     describe '#initialize' do
       subject(:cli) { CLI.new(arguments) }
 
       it 'assigns $stdout as the default standard output' do
         expect(cli.stdout).to be $stdout
       end
+
+      context 'without arguments' do
+        let(:arguments) { [] }
+
+        specify { expect { cli }.to raise_error described_class::ArgumentError }
+      end
     end
 
     describe '#arguments' do
@@ -32,11 +70,6 @@ module Producer::Core
     end
 
     describe '#run!' do
-      it 'checks the arguments' do
-        expect(cli).to receive :check_arguments!
-        cli.run!
-      end
-
       it 'processes the tasks with the interpreter' do
         allow(cli.recipe).to receive(:tasks) { [:some_task] }
         expect(cli.interpreter).to receive(:process).with [:some_task]
@@ -44,29 +77,6 @@ module Producer::Core
       end
     end
 
-    describe '#check_arguments!' do
-      context 'when recipe argument is provided' do
-        it 'does not raise any error' do
-          expect { cli.check_arguments! }.to_not raise_error
-        end
-      end
-
-      context 'when recipe argument is missing' do
-        let(:arguments) { [] }
-
-        it 'exits with a return status of 64' do
-          expect { cli.check_arguments! }.to raise_error(SystemExit) { |e|
-            expect(e.status).to eq 64
-          }
-        end
-
-        it 'prints the usage' do
-          trap_exit { cli.check_arguments! }
-          expect(stdout.string).to match /\AUsage: .+/
-        end
-      end
-    end
-
     describe '#env' do
       it 'builds an environment with the current recipe' do
         expect(Env).to receive :new