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/cli_spec.rb

@@ -89,11 +89,13 @@ module Producer::Core
             }
         end
 
-        it 'prints the error' do
-          begin
-            cli.evaluate_recipe_file
-          rescue SystemExit
-          end
+        def trap_exit
+          yield
+        rescue SystemExit
+        end
+
+        it 'prints the specific error' do
+          trap_exit { cli.evaluate_recipe_file }
           expect(stdout.string).to match(/
             \A
             #{recipe_file}:4:
@@ -101,6 +103,11 @@ module Producer::Core
             invalid\srecipe\skeyword\s`invalid_keyword'
           /x)
         end
+
+        it 'excludes producer own source code from the error backtrace' do
+          trap_exit { cli.evaluate_recipe_file }
+          expect(stdout.string).to_not match /\/producer-core\//
+        end
       end
     end
   end