Improve error reporting during recipe evaluation

* Report invalid action usages from tasks;
* Implement backtrace cleaning in CLI;
* Extract error class declarations in a new errors file;
* Replace raise with fail keyword in task DSL class.

spec/producer/core/task/dsl_spec.rb

@@ -12,6 +12,14 @@ module Producer::Core
         expect { dsl.evaluate(env) }
           .to raise_error(RuntimeError, 'error from task')
       end
+
+      context 'when given block is invalid' do
+        it 'raises a TaskEvaluationError on NameError' do
+          dsl = Task::DSL.new { invalid_action }
+          expect { dsl.evaluate(env) }
+            .to raise_error(TaskEvaluationError)
+        end
+      end
     end
 
     describe '#condition' do