Improve exceptions usage in specs:

* Throw symbols instead of raising exceptions to test code evaluation,
  this will reduce "bad" usages of exceptions, symbols seems a little
  better for now;
* Add a specific exception to be used in specs for testing behaviour
  when an exception is raised from a recipe.

spec/spec_helper.rb

@@ -2,3 +2,8 @@ require 'producer/core'
 
 require 'support/exit_helpers'
 require 'support/fixtures_helpers'
+
+
+# Specific error thrown in the error fixture recipe, we can't define it in the
+# recipe, rspec wouldn't know about it.
+SomeErrorInRecipe = Class.new(RuntimeError)